I have an IBM T20 - PIII 700/SpeedStep, 256 MB, 20 GB, 3COM miniPCI NIC, S3 Savage/IX (damnit, no GF2G), etc. - and XP has installed and run very well indeed since release.
The out-of-the-box drivers found all the hardware fine, and there have been regular updates of drivers for graphics, network and other bits and bats through Windows Update. Stability and app compatibility is superb, and the way it manages laptop things such as battery management, standby and hibernation is far better than Win2K did, IMHO.
One gripe - I use a Nokia 6210 as a modem thorugh the IR port. When XP sees the phone with IR on, it is there as a modem, and I can dial out fine. Problem is, each time I connect with the phone, it defaults to 9.6kbps GSM. I have HSCSD, and when I change the modem settings to V.110 I can connect at 38kbps or even 43.2kbps. Every time I set thism every time it works fine, and every time when I take the phone away the settings are forgotten, as the modem disappears from the Modems part of Control Panel. A small pain, but enough to be annoying... anyone with ideas?