No, this is not about the Canon mirrorless camera. Sure there will be more people knowing the EOS R6 than the actual subject of this post, but let that be a lesson for Canon to not re-use names from others. The R6 already existed (as did the R5 and whatever else they have in mind, only R1 and R2 are unused).
I’ve shortly mentioned this R6 in the past; this time I’d like to dive a bit deeper about its virtues and shortcomings for my uses.
In a much older post, I mentioned owning a Leica R6 with a rather comprehensive set of lenses. Actually owning two R6 bodies. In that post, I promised to dive deeper into it, and this is the start of that.
What is it?
For those that really believe the R6 is a mirrorless camera with the Canon RF mount… The R6 is a fully mechanical SLR made by Leica between 1988 and 1992. There is a later revision, the R6.2, which marks their last mechanical SLR. This means the only thing in the R6 for which it needs batteries is the light meter. It has shutterspeeds up to 1/1000th (the R6.2 goes up to 1/2000th of a second).
Such specifications are pretty normal for cameras made in the 70s and early 80s. But in 1988, it was a bit an anachronism. Perhaps also typical Leica; instead of trying to compete with the wave of increasingly competent and complex AF cameras, it went one step back to a more purist camera.

Is it any good?
In very short: yes.
But this isn’t a short story. If you google for this camera, you will find plenty of differing opinions and debate. Some arguments in favour of this camera, or against it, are based on sentiments. Many are about preference, rather than facts. Some are triggered by the brandname alone.
My opinion about this camera is also based on personal sentiments. While I’ll try to be as level-headed as possible, this piece too will just add the smorgasbord of opinions.
In this previous post, I already mentioned how the Leica brand will divide opinions. So I am not going to repeat that.
It’s not Leica enough
One frequent remark about the whole range of Leica R-cameras is that they’re based on Minolta cameras. For the purist who wants Leica to be synonymous with hand-crafted German quality: an abomination.
It is true to a certain level. The R3 was a co-development with Minolta, and was very similar to the Minolta XE. The R4 up to R7 shared a chassis based on the Minolta XD-7. Those Minolta cameras are good quality items. So why there would be anything wrong with that, is beyond me. The body is sturdy, solid, compact and comfortable to hold.
The lens-mount is unique to Leica. Some of the Leica R lenses were also shared Minolta designs (like the Elmarit-R 24mm), but many were pure Leica designs, like the Summicron-R 50mm and 35mm lenses. The lenses were made in the same sites as other Leica lenses of that era (Germany and Canada).
The inner mechanics of the R6 had nothing in common with Minolta. This model was completely a Leica-only affair. Whether that was the reason why it was far more expensive when new, I do not know. It was, though.

All Mechanical
Another point is whether a mechnical camera is any better than an electronic one. This is a discussion that is independent of the R6; you can have the exact same argument on the Nikon FM versus the FE. Is mechanical better?

As always, the answer is not a simple yes or no. There are positives to both sides. The electronic cameras allow more automation, like a shutterspeed priority or aperture priority mode. They often have more shutterspeeds. For example, the Leica R7 (which I also owned) had also half-stop speeds (i.e. between 1/60 sec. and 1/125 sec. there was also 1/90 sec.). It also offered aperture-priority mode.
At the downside, the electronic cameras do require batteries to work. If the batteries run out, the device stops working. And electronics in the earlier days weren’t quite seen as equally reliable – though that was assumption rather than fact.

Mechanical cameras can work without battery. If mechnical cameras start going wrong, it’s usually more a case of lubing and adjusting the mechanicals. With electronics, it easily may mean the end of it.
Mechanical cameras are often seen as more reliable, robust and durable. Whether that’s really the case or not – no idea. It does cause the mechanical cameras to be more expensive. Just as a Nikon FM2 fetches more than a FE2, so a R6 fetches more than a R5 or R7.
Two things remain though: battery independent operation, at the cost of having less automation. Mechnical cameras may work all the time, they will also always require you to do all the work: focus, set exposure and advance film.

A weird ‘tweener
Overall, the Leica SLRs aren’t as loved as the rangefinders. It’s the Leica M cameras that define their fame and reputation, not their SLRs. The first Leica SLRs (Leicaflex, Leicaflex SL and Leicaflex SL2) were complete Leica products. All metal, all mechanical, and rather big and heavy. The R-series were, as mentioned, done in cooperation with Minolta to rationalise a bit. This puts them in a strange position.
They’re not the Leica M, so not as desirable. They’re not the indestructable Leicaflexes – by reputation anyway. The advantage of this is that they’re relatively affordable.
For a Leica. Compared to other brands, they’re still quite expensive. And those brands that made SLR systems their main systems (Pentax, Nikon, Olympus, Canon) offer more choice in lenses, more complete systems overall.
While all these other brands were transitioning to all electronic auto-focus cameras, Leica decided to come out with the R6. Pretend it is far more reliable, and price it up there with the professional cameras of that day. It is a bit a weird camera.

So can it justify its high price tag? Why would one go for a Leica R if for less money you can get a similar specified camera with more choice in lenses for less money too? Why would you get one?
My take on the R6
Why did I get one? The badge. This I explained the post already linked to. I was curious whether the reputation was justified. Once I had it, I used it a lot. I like using it, except for one detail I’ll get to.
The good
By far the single best feature of the Leica R6 is its lens mount. Most of the Leica R lenses are excellent. I already stated the Summicron-R 50mm is the best 50mm for any SLR that I know of. The 35mm lens is a gem, the 60mm macro excellent. The Summicron-R 90mm a great portrait tool. I cannot really assess the Elmar-R 180mm and Elmarit-R 24mm as I never used them enough. But they never really disappointed. The lenses are relatively expensive, but the quality is definitely there.
The second best feature on the R6 is the metering. It’s a terribly reliable light meter, yielding very consistent results. In case of complicated light, there was spot-metering too. Switching between the two was easy too. Really a great implementation.
It is a relatively small camera, and operation is quite quiet. No loud mirror slap at all. This makes it a really discrete camera to use.
Last, the film forward: silky smooth. Quick to do, resistance felt perfectly weighted to me. Just really really good. Not the most important feature to have perhaps, but it makes using the camera nicer.

The indifferent
In a number of ways, I also find it nothing special. Build quality is good, feels solid and sorted. But nothing spectacular, lots of plastic. Nothing bad about plastic, but it doesn’t transmit a luxury feel. The camera does command luxury pricing.
The viewfinder is very far from bad. It’s not the largest I ever used, but not far off. It is not very bright, but definitely bright enough. You can change the focus screen; I mainly used the split-prism default one, as I like it most. But there is choice for different applications. Nice feature, but quite common for cameras like this, and still some way off from completely interchangable prisms like the Nikon F2 and F3 for example offer.
Specifications for shutterspeeds are ho-hum. A top speed of 1/1000th wasn’t bad, but the Nikon FM2, introduced 6 years earlier, already did 1/4000th.
It’s all mechanical. I happen to like it and my experiences with durability and dependability of mechanical cameras are better. Scientifically a worthless observation, I know. But I like it.

The bad
The shutter release. Not sure how to describe it, but the button itself feels like it has a very long travel, and/or the reaction to pressing it feels sluggish. It is as if there always is a delay between pressing the button, and the photo actually being taken.
For the record, I have had two R6 bodies, and both suffered the same. It is a complaint I read frequently from other owners. So I do expect the one negative (for me) to be generic for the R6, not just for my copies.
It doesn’t seem a big issue; especially not for the type of photography I tend to do which is quite slow-paced anyway. And objectively, it isn’t a big issue. But it feels off. It makes the camera less fun to use than it could be. You associate Leica with street and documentary photography, where immediate reaction is needed. This camera feels like it is not delivering that.
Such a stupid little thing, but to me really hugely annoying. Especially since the R3 and R7 I had did not suffer it. Yes, I still preferred the R6 over those two cameras, but still…. the brandname and pricetag make it harder to accept these niggles.

Conclusion
Is that one small niggle about a sluggish shutter release enough to condemn this camera? No, sure not. But this camera does not exist in a vacuum. As a SLR camera, it faces stiff competition from all other SLRs. The Leica M system has it a bit easier in that respect.
As it is, there are cameras available with similar specs for a lot less money, which are at least as reliable, usable and are a joy to use. For me, the Nikon FM2 has a leg up over this camera in nearly all ways but two. It’s a bit noisier, and its lens mount is not the Leica R mount.
To me, the reason to have a Leica R6 are the lenses. Beyond that, it is a very well-rounded camera but not very special. It makes it hard to defend its value-for-money.
My reason to sell my entire Leica R system was not just based on a flawed shutter release button. I felt I had to make a choice and keep just one SLR system. Both my Leica R and Nikon systems were similarly rounded collections for my needs. The Nikon lenses I can share with my DSLR. While that is a plus, it wasn’t essential to me, so it wasn’t the deciding factor. For both systems I had lenses I love, so in both cases I had to accept selling lenses I deeply liked. Still a draw….
But then it came to really enjoy using the system…. the body, its usability and haptics are very important. They’re integral part of the act of making photos, and the joy I get out of that (or not). And then, all of a sudden, a sluggish shutter release button can become a very big thing. It’s not fair, maybe. But every time I pick up my FM, or the Leica M4 for that matter, I just know they’re better cameras for me.

[Edit, made much much later]:
My, I put it down clumsy. “Is that one small niggle about a sluggish shutter release enough to condemn this camera? No, sure not.“.
Uhm. Yes, sure. It is. Everything following that sentence does nothing but saying it was my reason. And it was. Just that small difference between good, and very good. It may not annoy others. For me, coming to choose between selling a FM2 or selling the R6, it played a substantial role.