All machines in my house except for one (and all machines at work - worldwide) run on XP. Reasonably stable OS. All machines in my house except for one would crater if you tried to run Vista on them. Not enough memory, not enough disk drive space, processor too slow, etc. The one running Vista Home Premium (the one I'm using right now) runs fine. Now, I have Office 2007 loaded on it, too. I've found applications that I would like to load on the new machine will not install or run under Vista. PITA.
Would I recommend 'upgrading' to Vista from XP? No, nyet, nunca! Prime rule of engineering - If it ain't broke, don't fix it. If XP is running fine, leave it alone. I haven't seen anything in Vista that is 'better' than XP for my needs.
I put Vista on the new machine as I expect it to be my prime machine for 5 to 6 years and I doubt XP will be supported that long. I don't have to shut down the XP machines, so applications that have to run on XP are still supported. Just wish the backup software with my Intel Network Storage System (on the LAN for backing up all the computers) would run under Vista. Otherwise I don't have anything pressing that won't run under Vista.
Just my experience. We won't be upgrading at work for a while, and then I expect it will be when we get new computers. My IBM T42 wouldn't be happy with Vista at all. Not enough memory, disk space or processor power.