I'd be very interested to hear [...] being too flakey to run Pocket FMS as anything other than a backup GPS but would prefer an all-in-one type solution a pda/phone might offer.
Phones/PDA etc are just like computers. Use it for one thing and they are very good, consistant and reliable. When you start to install multiple programs, and use the device for lots of different functions then things start to go wrong.
Personally I don't recommond running PFMS on a phone, though many people do. They generally aren't designed for software like PFMS. Some problems are irregular screen shapes, small screens, no touch screen, and slower processors.
On the other hand PDA's which are not used for multiple functions...go get it and just put PFMS onto it, tend to work very well, and are very reliable.
Car Sat Nav's in my experience are the most reliable. Not all are compatable, but the ones that are work very well. I'm a beta tester for PFMS, and as such am constantly using versions that are under development as we try to get the bugs out. Even with beta versions, locks ups are almost non existant on a car sat nav. (I use the Mio C320 linked to above).
The big advantage is the cost of updating the units over a Garmin (software constantly updated, so you don't have to buy new hardware), having constantly up to date data, and having worldwide data (so you can easily bring it on holidays).
I would generally avoid phones, unless you've seen it running on one that you are happy with.
dp