My normal GPS is a Garmin GPSIII Pilot but I have also been experimenting with software on a PDA. I have an HP4150 and have loaded this with both the Tom Tom road software and PocketFMS. Both work extremely well using the Tom Tom bluetooth GPS receiver.
It was not all plain sailing though. It took some time to get Pocket FMS working properly - this involved a Registry tweak to get it to lock onto the GPS receiver.
Whopity is right on a couple of points. The PDA screen can be pretty awful in direct sunlight. Avoid this and the display is good - twice the size of the Garmin display and in colour too! For some screenshots see
here. Yep, the article is aimed at users of FS2004 but don't be fooled as I have used the PDA in our Citation flights too.
You will also have to seriously think about cost. If you are trying to pick up a PDA cheaply you will still have to budget some serious money for the road software, GPS receiver, a memory card (ideally 512Mb) and the aviation software.