Sorry, I don't want to flame, but I agree with arran. I can understand why you might like to use these other renderers and I think its good that you are experimenting with them, but how can you be bored with Mental Ray??? Surely you can't know absolutely everything about it, let alone enough to be bored with it? Personally I don't think the renders that it produces are that great, fast render times or not. Mental Ray and PRman are pretty much industry standard because they are good, I'm sure that if there was better out there all these post-prod houses would be using them...

Its great that your're experimenting though, whatever floats your boat :p