Every time I think about it...and its often.... I cannot come up with a good reason to ditch OS X for linux...in other words they are so closely related that I feel like I get all the benefits of linux in OS X plus a UI that I quite like...
I will have to say that baryl and compiz are pretty slick though, desktop effects like that are tempting indeed!

On the windows side it wasn't a hard choice at all... I have a vista box (which I'll keep) and a 2003 domain controller (which I'm replacing currently) and other than that I've moved all my formally-windows hardware to linux and never looked back.