I think it's m$'s best OS, but I ultimately dislike it for the following reasons:
1. It absolutely sucks on a computer older than 1 year.
2. Impacts raw performance (which sucks for me because I use windows ONLY for games).
3. They tried too hard to organize things (example: the control panel sucks.)
4. It just requires too many tweaks to make it functional (have to disable this and that, like driver signing and UAC and enable this and that like admin, etc.), and this gives me hell because I reformat windows once every 2-4 months (XP takes me long enough to get setup after a format, 4 hours or so; I don't need vista making that longer)
So basically if you're just building a computer and are going to make it fairly modern, vista could be a good investment. But if you're like me and are out for raw performance and don't depend on windows for regular desktop work, XP is probably better (or even 2000). For desktop use, a lower-maintenance OS is what I prefer.