My HP laptop (tx1210) runs Vista Home Premium, and I'm constantly performing actions on it that pop up the User Access Control window. I don't have a problem with this in and of itself, as I've read Microsoft's side of the argument and I'm willing to give them the benefit of the doubt and assume that this does actually provide some additional security.
That said, it's slow as hell.
The UAC popup isn't bad on my monster desktop, which has a top-end Core 2 Duo, 4 GB of RAM, and a decent video card, but it takes FOREVER on my laptop - probably 3-5 seconds to black out the screen and pop up the prompt, and another 3-5 to black out the screen again and return me to what I was doing. This is unacceptable in 2008.
After reading a few unhelpful posts directing me to disable UAC completely, I found one that mentioned that it wasn't as slow with Aero (the pretty transparency and other visual effects package in the higher versions of Vista) disabled. So I opened the Performance Information & Tools feature in the Control Panel and selected Adjust Visual Effects. After turning off the vast majority (I left Use visual styles on windows and buttons because I just couldn't bear to look at the old grey XP-style taskbar any more), guess what? UAC is now lightning-fast and doesn't bother me at all.
I'll probably try re-enabling a few more settings and find the balance between speed and graphical sugar, but disabling most of the visual effects seems to have done the trick, and is already improving my Vista experience dramatically.