I have been using Vista for a while -- it came with my new PC, Vista Home Premium. It took some time to get used to it and it's quite good. But I do sometimes want to go back to XP, when it is slower than XP, which I use at work, and my notebooks. Well, I was too lazy to install XP over Vista, so I kept on using it. There are just too much stuff on there, and I don't have time to back up and install new OS, reinstall everything, and restore, so on.
So today, some software I need to use doesn't work with UAC, or I have to buy the latest version just to run on the new OS. Heck, I always thought UAC is a good feature, but to run that piece of software which I don't think it worth the money just to run on Vista. So I turned off UAC completely. (With Vista Home Premium, it only allows either turn it off completely, or turn it on.)
I don't know if this is just my imagination, but it feels like the computer is running a lot faster, even all the visual effects are turned on. Well, for now, I'm keeping it off.