Very few problems here.
Mouse used to occasionally freeze at boot, but I guess some update cured it as it's fine now. Only current problems are that the sidebar won't display if I start it with windows (it is running, though) and I have a similar problem with WMP not showing the video occasionally (though I think the latter is most likely due to the recent DivX update); also won't print from Word 2007. Haven't had time to investigate those 3, but they were all originally working fine. I can't remember if they stopped working properly at the same time as SP1...
Mind you, I did download something to tone down UAC's constant questioning of whether I REALLY want to do something!