I installed XP SP 2 RC1 on my primary laptop about a month ago now.  WOW! ... I was initially hesitant ... as i really didn't need another rebuild due to something it might do to my machine.  This is the best SP i have even seen... i have not had a single problem ... i connect to many wireless networks as well as hard wired ones and the updates to the Windows Firewall is simply amazing.  IE changes work really well ... the security center in control panel makes it easy to see what you have configured etc...

I think the best parts of this SP are the changes to IE.  There are now much more meaningful dialogs when you download stuff ... and when you run stuff. 

I know what i am opening myself up to when i download things and run stuff from the net ... but the majority of people in this world don't have a clue ... and i think these changes will make a huge difference for the average user in terms of keeping their computer free of viruses etc...

I am truly amazed at the quality of RC1 and would highly recommend it to anyone.

For heaps of information on SP2 see here:

http://www.microsoft.com/technet/prodtechnol/winxppro/sp2preview.mspx