I've had the R3a for almost 10 years now, and it was used then. As i'm sure you know, same camera essentially save for AE/electronic shutter/'suggested-versus-selected' meter readout (which I'm on the fence about: I prefer match-needle but like seeing my shutter speed).
With some metal in it, it feels a little more solid than Cosina's SLRs (Nikon FM-10); a bit more like the old Canon Elan 7. The rest is plastic, and mine has taken a few dents and abrasions that don't look as graceful as a Leica, but it survives. The back, film advance, and rewind aren't nearly as smooth as some vintage camera, but they work fine. Personally, I love the folding rewind crank: looks like an M3, functions like an M4.
Metering pattern is a little weird, a sort of very oblong, bottom-center-weighted.
See this thread.
Shutter is loud, but who cares? The recoil is very soft and easy to handhold at slow speeds. The release is convex, and I find it binds a bit with a soft release—increased leverage=more play, but naked, it's basically its own soft release.
The RF/VF are what shines on the R3 though. 1x magnification and very bright, much more so than my M5. Despite being left-handed, I'm right-eye dominant and love shooting both eyes open. The patch appears to be a bit bigger than the M5 and crisper around the edges, though it flares some, and the RF window is in prime position to be covered by fingers. The baseline is long enough, and the VF big enough, that shooting a 90 isn't a problem. Want to go wide? go with accessory finders.
Framelines have parallax compensation, just like the Ms (and a few old compacts, like the Yashica Electro). To my eyes, the 40 frame line is a bit hard to see without looking around the finder, but I shoot with a 35 anyway and just guess at it. No auto selection, which means sometimes forgetting to switch them when changing lenses, but they're marked so you know what youre seeing.
Mine has come out of adjustment a couple of times, and fixing that isn't always for the faint of heart. The flash shoe needs to come off, and then you're faced with two VERY tiny screws and one hard to reach bigger one, deep in the body. My first attempt resulted in me stripping, and then unscrewing the vertical screw entirely and an expensive trip to Stephen Gandy.
There's some great accessories to be found, like the Leicavit-style Rapidwinder, a right-hand grip (that doesn't replace a bottom plate!), and VF eyepieces.
New, I felt these were a bit pricey for the build quality, but what you're paying for is that beautiful finder. With the new MP at almost $5k, it's a great bargain to be able to shoot Leica glass and a shame they were discontinued. Stephen still is the go-to for fixing them.
I'm realizing that little of this answered your question. Yes, they're reliable. Plastic cameras bounce, rather than crush. Very little electronics to go out of whack; maybe your batteries won't like cold, but the mechanical shutter won't mind. Metal blade shutters seem to stay in sync a lot better than cloth ones. The RF alignment always makes me nervous that it might go cattywompus on a trip, but that's inherent of all RFs. I've had mine for 10 years and its needed adjustment only twice. All the mechanicals dealing with film have been perfect.
Best of all, it's very light, so less energy when it does inevitably impact something.
As a personal testimonial: It's my go-to for hiking, clad only in a Domke wrap, including a year of weekly trips to Snoqualmie National Forest and Olympic NP—not walks in the park!
