It's just too annoying to use. Either software makers need to start digitally signing everything or we're going to have to click about a billion, zillion times to get through every UAC dialog for everything.
It hasn't been too awful to deal with at home. At home UAC is rarely getting in the way. However, at work today I got the chance to install the RTM on my second machine. I ran my setup scripts that I distribute for project setup. While it helped me find a few bugs in the setup scripts, I had to enter my password (because my machine is part of a domain, UAC requires a password, not just a click through), at least 15 times.
UAC's a good idea, but I don't think it came together in Vista and should probably just go away. If we have to click for everything that might possibly compromise our system, coming from the way XP treated binaries, we'll be clicking very often (as I found out today).