I've been using Mental Ray for about 8 years now, and still struggle with the damned thing. It's a great renderer, but the integration in Maya is appalling. The Max integration is better, but still not great. My favorite renderer is still Brazil R/S for 3ds Max; but it hasn't been updated in a very long time. The company was bought, and it's turned into vapour-ware.

From what I've played around with Vray in Maya, I like it quite a bit. It has it's quirks, but it has a much better workflow in Maya than MR has. It's SSS shader is pretty good too. Simple, and fast.

Still waiting for Arnold though