I have two of the older SLRs, an SL and an SL2; have never tried any of the Rs. The SL2 came with the 50 Summilux; I have the 50 Summicron for the SL and several of the "cheapest" of the lenses, all the 2-cam versions with one exception: 35/2.8, 90/2.8, 135/2.8 (3-cam) and 180/4. None of these was unreasonably priced, but of course they're '60s-'70s designs. However, they are quite good, as you'd expect -- the equivalent optically of the corresponding M lenses. (This opinion is probably not shared by all Leica fans!)
The Leicaflex bodies are quite hefty, and certainly not particularly feature-packed -- just basic match-needle SLRs, albeit with semi-spot metering, which is a little unusual. But that's about what you'd expect from a company that didn't even have a metered rangefinder model until the early '70s (the M5). The SL2 is quite refined, rather like the M5, which was made around the same time. But basically you bought these "diesel Leicas" for the lenses; Leica couldn't offer anything like the system capabilities of Nikon and Canon.
They are very intuitive to use (perhaps with the exception of the meter), and the shutter is really nice indeed -- a soft "fwuff" rather than the harsher sounds of the better-known SLRs. Definitely well worth trying out. I should mention that the cameras take mercury batteries, so you will need to use one of the various workarounds unless you have a hoard of them still.
Doug Herr's website is a trove of information about all the Leica SLR cameras and lenses. I highly recommend it. I think it's by subscription only now, but he is a great guy and very helpful.