The only OS I own is XP but I've been testing the RC of Windows 7 and thanks to Microsoft's technet blunder, Vista Ultimate.
I have to admit I was biased against Vista after being an XP semi-fanboy, this was partly due to my first experience of it being buying an ultra-budget laptop that came with it and was far too crap to handle it, and partly because I felt it simply wouldn't bring anything I would actually use. However, I now see there's nothing wrong with it at all, it's a perfectly sound OS.
Win 7 has the nice shiny interface but there are quite a lot of annoying minor changes that come with it, although you can disable/tweak most. I suppose it's "the future" but meh. Personally I can't see what 7 does over Vista for my personal use, but I'm sure there's a ton of improvements that you can't "see" at first.
I suppose forking out for a genuine copy of Vista at this stage would be pointless; whether I buy 7 all depends on how much they charge for it.