Been using it since late last year. Nothing much worth noting from the average daily use perspective. As thephilosophizer has pointed out, outside of a few very specific features, it is largely a rollup of previous patches. Saves on upgrade time if nothing else.
Business and especially enterprise-class customers could need further evaluation testing, but otherwise, I do not see much of a reason for the average WinXP desktop user not to deploy SP3 at this time.