I have UAC off on my main system. As I keep doing stuff that makes it pop up.
But on my laptop that I just bought I kept it on. I don't do that much to it that triggers the UAC.
Then there's the issue of compiling DX gadgets - I'd expect that gadgets compiled with DX3.5 will have problems with UAC? That'd be a problem when sharing them to other users.
DX4 seemed to not have issues with UAC though.