Well after having a laptop with Vista on it for nine months, I can confirm it was a very big change. Antivirus, firewall, DVD/CD creation software, all would not install/work correctly under Vista. Even with the compatibility software running.
But I found substitutes for all of the above, and it was just a matter of adapting to them. It's a shame, though, that several old games would not run at all (but when XP came out there were significatly more games that were incompatible!!)
And in reliability, Vista is streets ahead of XP (maybe because it's so fussy about what software you can use.)
I personally like Vista, I don't regret upgrading, I just wish I'd waited until now, because more and more programs are now compatible. I haven't met with any problems at all, just to comment that like any PC, a clean reinstall every 6 months speeds things up a lot.
But I won't be upgrading any of my other manchines to Vista yet, there's no need, the laptop is the only one I use on a daily basis, and for daily use, Vista is SO much better.
My favorite thing is the way you can hit the 'Start' button, start typing the name of a program and it's icon appears straightaway.
Don't believe the critics 'till you've tried it yourself.