You won't regret it, in fact you're going to be happy to leave behind the abominable thing that mental ray is.
The only thing is that rendering volumetric spots can't be faked in the same way you can do with some of the older engines. You'll have to either render true volumetrics which can slow down rendering a bit (but still deliver fantastic results) or use some fakes like illuminated cone objects in the light's spot direction that need some special attention. I'd say test this and see how it works for you.
But you'll be much closer to photorealism right from the start and probably way faster than with mental ray, in regards to both rendertimes and turnarounds/lookdev with interactive rendering.