I have installed SP2 on 3 different clients servers. The best feature hands down is the increase in size of the Information store to 75 gig. I haven't had a chance to play with the Mobile stuff YET because my Axim, Active Sync, and WM5 are all weird together.
As for XP clients to the Exchange server, haven't had any problems or calls from clients. I would say make sure you are all up to date for Office XP.
Also just in case you were not aware, since you have Exchange 2003, you are entitled to install Outlook 2003 on all clients that will accept it. (no Win98 or lower)
Good luck... Exchange can give headaches no matter how awesome it runs any other time.
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