I've worked on both mac, windows, Gnome and KDE - and used them all a fair bit.
I have to say the mac style really isn't my thing - sure it looks really cool, but searching around on the dock to find the one application I want out of however many really isn't my cup of tea. It's all visual based - I prefer good old text I can look at from the word go. Now I'm not a Microsoft fanboy by any means, but I do prefer the environment on a windows machine (at least with XP, I wouldn't say I've used Vista or 7 enough to make an informed, unbiased decision.)
However, I do prefer both KDE and Gnome to both of them. Ask me this question 2 years ago and I would've been KDE all the way - however now I am more of a Gnome boy. Truth is, I'm happy working with either of them - ctrl+alt+arrow keys to switch between one of 4 desktops is my preferred style. It's like working with multiple monitors, once you get used to having 2 just having one seems so restrictive.