I've been using a Vista system for the past couple of weeks, and in my limited time I've come to the realization that it pretty much sucks. Well, I actually realized this roughly two hours after I got the system, but regardless, do NOT upgrade from XP to Vista! Mark my words, you will be left with a big sad face. Here's why:

1. Compatibility - A lot of programs that used to work in XP don't work in Vista. If you do anything other than web browsing/chatting/Office stuff on your PC, be warned. Photo/Video/Music editing software probably won't work correctly when you upgrade to Vista.
2. Boot Up - Microsoft is fooling you. You get to the desktop relatively quickly, but it still takes like 3 more minutes for all the startup programs to actually startup. This might be specific to my system, but nevertheless, XP was much quicker to start up.
3. Sloooooooooow - With a new OS you'd assume that they would have optimized Windows to make everything quick. The first thing I tried to do once I started the system was to copy some files from my flash drive, and unzip them. First of all Vista asks you whether you want to "fix" your flash drive, although it works fine on both Windows/Macs, and then it takes like 30 seconds to even start copying files. The built-in unzip tool in Vista is pretty ridiculous too. It estimated that it would take 45 minutes to unzip one directory. Winzip running in XP took 30 seconds. WHY?!?!? When I try to preview pictures using Windows Picture Viewer, they take like 30 seconds to load. WHY?!?!?
   
4. UAC - User Access Control. I think the Mac commercial below really sums up this "feature".
5. Buggy - Even though MS delayed Vista's release by a couple of months to fix some bugs, it still feels like an unfinished product. Again, these might just be issues on my system, but for example when I close my laptop and open it up again, the display doesn't turn back on. Also, I have to manually connect to an access point every time my system starts up. Why does this happen?

One somewhat good thing about Vista is that it looks a little nicer than XP. But if you don't care about all the gimmicks about Aero and crap then this isn't even a good thing either. Don't buy Vista. Buy XP/Mac. Now watch this commercial, it's funny.