I think it was ArsTechica who did extensive gaming review under Parallels6 and A LOT of games were totally playable.
There are few somewhat older or less performance heavy games that would work perfectly well under Parallels and that's a big benefit. I don't want to switch to Windows every time I want to play something; but if I know my system is powerful enough to run a VM and play the game, then why not?
Sometimes you can get away with making your own Cider/CXEX port or using Crossover Games or something, but running VM on top of a bootcamp partition seems like an easy solution.
I use VirtualBox for virtualizing linux; it works great; but they don't care for 3D graphics support. But Parallels folks do as that's one of their bigger selling points.
I did this video myself:
http://www.youtube.com/watch?v=g6TsCAPPqIU