Iv'e been using Gentoo on my servers for many years now and I can't even imagine trading that for even another Linux flavor. No other system gives me the flexibility I need for development and the power tools to back it with, it is BY FAR the best tool for my job, the way I see it.
That said, my desktop environments HAVE to just work, I learned that the hard way after fighting a rather ugly system upgrade my Gentoo desktop required after I've mistakenly updated a configuration file - which cost me an expensive half day of work.
That's why on my desktop there's only one choice - Kubuntu. For me it's not plausible to use OSX or Windows, they just don't do the stuff I need them to do and their UI doesn't and CAN NOT (nor is it designed to) work the way I want to work. KDE4 on Ubuntu makes the most sense. Everything hardware-wise just works perfectly, package management is a breeze and KDE is the right power tool for the job.
I'm sorry, but as much as I try I just can't figure out how OSX is an acceptable solution. Other than utilizing the hardware right it pretty much does everything WRONG for me.