I am using XP Pro build 2600- It is fine so far. My CH Fighterstick and Pro Throttle don't have drivers, but they do work (I just can't program them with speedkeys anymore).
I have an HP OfficeJet 710 (scanner, fax, printer combo) that will not work with the XP. There are no drivers available to make it work either.
My adaptec CD software (version 3.0) won't work, but to a point that is immaterial. XP has built-in support for CD burning.
I have yet to experience any serious problems (really, anything other than what I have already mentioned).