I quite like GI Joe. I haven't really delved properly into mental ray/global illumination (I'm not really much of a lighting person), but I did the work in this thread using GI Joe:
I found it quite easy to get the effect I was looking for, and I was pleased with the render considering I didn't go down the now usual Mental Ray route. As well as using the normal GI Joe setup, I played around with the settings, added a few of my own lights and rendered in passes to get more control in the end, so you can do more with it than just pressing the button and rendering away. I think it does have more merit than being just used for 'clay renders'. Using it does make you think about your lighting a bit more, rather than just checking on 'use final gather' in render globals.
You will probaly need to mess around with all of the settings, and do lots of test renders to see what the effects are on your scene.