I've just finished upgrading both my TabletPC and XPPro Desktop home PCs and my the XPPro desktop in my office with Service Pack 2 (sp2) so I thought I'd share my experience since I'd yet to read anyone else's online and I know that there are plenty of folks out there waiting for the guinea pigs to report before doing it themselves.
Downloading
I got impatient and didn't wait for the "automatic update" feature of WindowsXP to download sp2 for me. Instead I stumbled over the Network Installation Package for IT Professionals and Developers version and downloaded it myself. This version is designed for those who will need to update multiple computers. (i.e. a library full of staff and public computers.) The only difference between this download and what you'll get via the auto update is that this is the complete package while the auto update will download only the files you need for that particular system. This version downloads all the files and the installs only the ones you need. At 272.391MB it's a hefty download. On my cable-modem it took about 20 minutes. For those using this version, download it once, place it on the network somewhere and install it from the single location on each machine. (That's what it's designed for.)
Installation
The installation itself took a total of just 16 minutes on my desktop. The uncompress and install itself took 11 minutes while the reboot and post-install functions took five minutes. (I forgot to time the install on my Tablet but it wasn't much longer than that. The additional time would be a result of the fact that the Tablet gets a few more updates than a regular desktop.)