Well, I also use Pocket FMS on a Dell Axim.
Advantages:
Full colour
Excellent integration with PC, so planning on PC is easy and straight forward
Full colour maps with gps trail can be printed off from the pc afterwards for passengers, as a record of their trip.
Map & Navigation data freely available for worldwide locations
Program being constantly updated with regard to users feedback and requests
Planning on the PDA has significantly improved, and is now quite easy.
Software is free.
Disadvantages:
PDA screens tend not to be as bright and clear in direct sunlight as a Garmin gps
Nav Data is not official, and can and does contain errors. Not that common, but there are some there. Therefore it must be used in conjunction with a chart
PDA's can crash and need to be rebooted. (Not much of an issue if you set it up on the ground, and leave it alone in the air, and simply follow the plan.) As the development of the program has gone on, this seems to have become less and less of an issue
Their source of the navigation data is closing down, and they are going to have to move to a user maintained database. We'll have to wait and see, to see how well this works.
On balance I think it's an excellent piece of software, who's biggest limitations come from the PDA itself. So long as you're aware of the limitations and weaknesses, they aren't really much of a problem.
dp