I installed this upgrade to Windows XP Friday night on one of my PCs.  Installation, including download time (cable modem), was about an hour and a half.

One of the first things you notice is the new security features, including firewall and popup blocker for Internet Explorer.  Both seem very well integrated.

So far no problems, and I'd recommend anyone who uses XP do the update.  If you're a dial-up user, you should be prepared to setup your computer to do the download and update overnight.  Better yet, sign up for a cable modem!  (DSL is the second-best choice.)