I am currently running Maya on my Powerbook, and things work beautifully. It's not as fast as running it on a comparable PC, but for me it works just fine. Functionally, Maya for OSX works the same as Maya for Windows (Maya Complete, that is, since there is no Unlimited for OSX). There are occassional display glitches, but I attribute that to Maya for OSX still being in its infancy. Overall, I am very happy with it. In fact, I use Maya on my Mac about 90% of the time -- the other 10% is on my PC when I need to use PC-specific exporters.

Bottom line is use whatever fits into your workflow. If I was getting a machine just for 3D and nothing else, then I would go for a PC. Maya is faster on it, you have many more options when it comes to 3rd party plugins/software and video cards, and PCs are less expensive. I'm running Maya on my Mac, because I personally prefer OSX over Windows, and I like the Powerbooks more than any PC laptop on the market, regardless of performance. That is just me, though.