On the plus side I have noticed quite a bit of the extra security - images blocked in spam emails to stop the sender identifying my machine, and it's popped up asking me to set rules for allowing stuff like MSN messenger to access the internet, which is all good.
Only downside spotted so far is that my desktop icons take slightly longer to appear properly on start-up, but not enough to be a problem.
Also took this as a good opportunity to have a defrag, and generally I'd say I'm happier since the SP2 update.