I have recently made an investment in the V-Ray render engine for 3ds Max. I was persuaded with promises of easier set-up, more material options, and faster, prettier renders.
Though "switch" may not be the right word (as I am much more familiar with Mental Ray and iRay and I can still do faster/more efficient work in them), I do expect to be utilizing V-Ray more in my 3D projects. It just seems to have more potential. Especially with its much friendlier atmospheric settings and interactions with FumeFX, it has been easier to get a desired result than with Mental Ray.
I have been pleasantly surprised by its performance so far. Neglecting the numerous errors and mishaps that have happened (chalking it up to my unfamiliarity with it), it has done a very good job at making really great scenes with much less time spent on materials and lighting at least. When it comes to render time, though, I have to say it isn't much faster that Mental Ray for the same size & quality. This may be because it isn't fully utilizing my processor during render time. For some inexplicable reason, it only uses, on average, 35% of the CPU... Obviously this seriously impacts render time.
Anyway, I have a ton of huge projects planned, so be expecting more posts, more frequently with higher-quality content!