I have two machines with WinXP Pro installed (And X4, too). On my backup machine, I have just installed SP3 (for WinXP) and I am making sure that everything remains stable. I will upgrade my main workhorse from SP2 to SP3, after I am satisfied that I will find no problems.
So far, so good. But I am still cautious.
EDIT:
One more thing, these are AMD Athlon CPU's, and there was a cautionary note about SP3 and AMD CPU's. Something about endless reboots instead of a normal startup. So far, I have not experienced that, but then again, my PC's are custom built and do not come from the manufacturers whose products exhibit this phenomenon.