Investment? No camera is a good investment -- you buy the tool, you use it, if you are fortunate you like it so much you wear it out. The return on investment is job satisfaction.
As to their usability -- Leica R models are like all Leicas -- built to last. I can highly recommend the Leicaflex SL2. The meter is highly accurate, the screen incredibly bright and easy to focus, the camera is built like a tank and the shutter is like good sex when it fires.