You should take a look at the topics torrentstorm and i have had on the matter in the past. We usuallt talk about it in an unbais way where only facts about the OS's are presented.
And i'm pretty certain (i'm too lazy to check up) that XP out does 7 in terms of most tasks. Its again (i hate repeating) the security thats different in Vista and 7 as the core has been changed with that in mind + alot of other features and changes but security is the big one.
@TS i agree with everything you said other than the fact that vista is not problematic, its not too bad anymore but when it came out it was screwed.
I have vista at home as well as XP, 7 and Red Hat i don't have any macs but i've used Leopard a bit and played with Snow Leopard. All of those OS's are nice for different things. I prefer 7 to all the others but thats just because it suits my work flow and how i organise work.