I'm using it on both XP pro and XP home (both SP2) and haven't had a problem yet.
EDIT
Or, at least, not a problem with running it. I'm still losing all my appointments whenever I re-start the phone though.
This has now become a huge issue for me. The phone does not appear to remember anything that is synched to it. Full stop. So, if I've entered 50 appointments, synched them with Outlook and then turn the phone off (like, for example, flying on a business trip) then I will have lost the lot when I turn it back on. This is using Outlook 2002 and Exchange 2003. I never had this problem with good old Lotus Notes or Lotus organiser.
For the last week I have had an XDA II on loan from my ocal telco (they will loan them out for a week to anyone who has a contract with them!) and it is not the lame-duck people have been making it out to be. The synch cradle charges direct from the USB cradle (so no need to keep a separate charger at work) and the battery easily lasted a full weekend - even though I was using it a lot (after all, I was playing.....errr....testing all the features). Plus, it didn't crash or lock up once and, as I say, I was messing about with all sorts of things and instaling trials of software left right and center.
Mobile IE is nowhere near as good as Opera, but Opera are about to release a version for PPC. The e-mail was very good, texting was fine, handwriting was excellent, calendar is good etc. I didn't find anything that I do with the P900 that the XDA II didn't cope with.
The phone is by no means as slick, but then, I use my P900 more as a PDA that can make phone calls anyway. Plus, the two absolute killers for the XDA II, I can add Wi-Fi to it and THE SYNCH SOFTWARE WORKS. I've spent the last eighteen months working around the problems in the PXXX's synch software, knowing full well that this was early technology and that SE would fix the problems ASAP. After all, the problems weren't there in the R380 so I knew it wasn't a problem with the underlying OS.
However, it now seems clear SE have no intention of fixing these problems. The third generation device has now launched and they are all still there, present and (in)correct. No categories, no repeating appointments, random lost contacts, frequent lost calendar entries. And, far from things getting better, having now been forced to switch from Lotus to Outlook, things have gotten worse.
The P900 is supposed to make my life easier. But there is just no way I can justify continuing to use a device whereby, every time I turn it off, I loose all my appointments! Sure, the XDAII isn't perfect. It's bigger, it's heavier, it doesn't have a flip, the battery doesn't last as long etc. etc. But, it manages to keep track of my calendar without bastardising it, and I can make and receive phone calls. Am I going mad in this glorious September sunshine or is the XDAII really a better bet than my P900? Someone tell me I'm certifiable?