I've got some pretty hefty Excel sheets on my iPaq, transferred over from the PC, and they all work fine. Even the several-megabyte Churchyard file (who's buried where, when, etc).
The "basic" Excel functions work in Pxl, but not much else. My Pilot's log from the PC went without a hitch; my Lat/Long to NGR converter laughed at me.
I wasn't impressed with the GPS sleeve - but others seem to like it.
There is a serious bug in the MS diary software in the iPaq, that caught me out. If you scroll back and forwards looking for a free day for something, it will after a couple of "passes" not display multi-day events (like being on holiday). So you book up something else...
I dumped the "provided" diary, and use Pocket Informant, which is brilliant.