eksasol: I never have problems with the Logitech MX518, G5 or G400s in term of buttons durability. Only problem with them is after a few years the laser goes haywire and stopped working properly, so I always keep an extra new one in stock. I'm not really into the design of the new Logitech mice (or Razr ones), so I'm considering Steelseries when my Logitech dies. I can't stand these gaming mice with rainbow LED that lights up the whole room, or a have a design that make me look like some l33t hax0r fps nerd, at least with the Steelseries I think I can turn them off.
I second the G5. Mine still works after more than 7 or 8 years, though the feet are so worn down that it's usable for precice movements. Never had mechanical issues with it, other than the mouse wheel clogging up so badly with dust and debris that scrolling stopped working and I had to take it apart and clean it. Workes like a charm.
Currently I'm using a G502, but I haven't used it long enough to make a definitive statement on it's reliablity. Generally though, I never had a Logitech product fail on me.