I installed it on one computer after a fresh install and had no problems. I think most problems that occur are due to conflicts with some software that was installed prior to the SP2 Ugrade, or due to malware on the system. Anyone thinking about upgrading should read through this thread first:
http://www.daniweb.com/techtalkforums/thread10031.html

I'd agree. If your system is in a mess, installing a Service pack won't 'fix' it. Clean the system up first, or format and start over. The Service pack is designed to strengthen Security measures, so you need to give it a chance to do so.

Software which gives problems afterwards may simply be 'badly behaved'. For some programs you need to set 'exceptions' to the security measures put in place.

Yeap, and rolling it out on all of the computers at work as they come in for cleaning

Upgraded without any problems... thankfully

Installed on one computer so far. Turned off the XP firewall since the computer has Zonealarm installed. Zonealarm brought up a number of alerts about changed programs (as expected) but no other problems so far.

I've had no problems with it and have it on quite a few (20+?) machines now. The only weird thing I've seen so far, is that on my machine when I sign into windows messenger in a particular way, it resets the font size on my tooltip boxes... very weird, but nothing really problematic.