I've had the chance to install the brand new Windows Vista - Ultimate edition on a PC with a P4 3.2 GHz processor, 2 GB RAM and a GF 6800GT graphics card. The installation process is very easy but also incredibly slow and worst of all the progress indicators leaves you in doubt of what is really going on. But well, it does install eventually.
The operating system often appeared slower than XP on my PC and I guess that it is due to all the unnecessary graphical features they have built in. I guess most of them can be switched off but I never really bothered to try.
The user interface has become very Macintosh inspired. It is obviously designed for computer novices and does everything possible to prevent the novice user from making mistakes. At the same time it does everything possible to prevent the expert user from enjoying the interaction. Even when/if you manage to get rid of all the most annoying confirmation boxes it will still be nagging you from time to time and be a pain in the ass.
Oh yeah, it's got some pretty neat features as well but not nearly enough to make up for all the functionality that annoyed me.
Device drivers for graphics card, sound card etc. are currently mostly beta versions with limited functionality. Utilities for your devices are for most parts not released yet. On top of that many of your XP programs will only run with compatibility problems that you may not notice until after a while.
My advice is to stay with XP until at least SP1 is released and all drivers and utilities are ready for download. Then give it a very good test first before you decide if you really want to upgrade or if you are better off staying with XP.