I've turned User Account Control (UAC) off in Vista due to the "never ending" troubles they cause, when you're trying to run Visual Studio, copy files from Inetpub, and the list goes on...
However - there might be another reason to disable UAC. According to Rick Strahl UAC might slow down your performance quite a bit, sometimes up to 2-3 times!! Check out this blog post from Rick's blog and decide for yourself.