Much like Win2k was the stepping stone to WinXP (they are, at their core, very much the same thing in most ways) Vista is just the stepping stone to the next OS release in 2009/2010. If you don't have some pressing need to upgrade to Vista (such as wanting DX10 support) you won't really be missing out on much overall if you wait until the next version.
I only upgraded because I like learning and tooling around with something new. I've got Vista performing at least as well as XP with most things, better in some cases, and worse in some involving dated software (including a few games). If you have the hardware, you're willing to learn what to disable, what to uninstall, and what to tweak--Vista is just fine (now that driver support has caught up quite well). Pretty much exactly the same as it was upgrading to XP in '02-03... 
I do wonder though--do enough people actually sit around all day searching their HDD for files they saved last week or email from grandma to even justify the existence of Windows Search? Does anyone actually read every one of the constant UAC pop-ups before just clicking "yes" right through them, and if so do they retain anything that resembles sanity? Thankfully things like these are easily disabled altogether--increasing performance and reducing annoyance at the same time!