Vista’s got a lot of bugs.  If you have a computer that’s less than 2 years old and you’ve been contemplating the “upgrade”, I wouldn’t do it.  The major problem right now is not entirely Microsoft’s fault, but as normally happens with any Microsoft update, there are many driver issues right now.  That is caused primarily by 3rd party hardware creators who haven’t created compatible drivers for their products.
There are some tremendous features in Vista that will eventually make the upgrade worthwhile.  The speech recognition is fantastic and has surpassed the few that I’ve tried in the past by leaps and bounds.  The security side of Vista works well, unless you turn it off, of course.  No matter what anyone else out there says, ReadyBoost works.  Aero is pretty, but a resource hog and I turned it off.  They added the Desktop to the Alt-Tab shortcut, although my shortcut was always to hit “Windows+M”.
I’m not using Vista for my primary computer yet, because I have too much 3rd party hardware that doesn’t work yet.  I have moved the family computer over because it’s a pretty basic machine that everyone uses, and I like the parental controls I can use with Alaina’s account.  Even though it’s a step down from my primary computer, it runs very smoothly.
When it comes time to purchase a new computer down the road, I’ll probably go with Vista… preferrably on a Mac with BootCamp+Parallels.