I've used both PDAs and dedicated devices. Advantages of each depend on your applications.
Advantages of PDAs over dedicated GPS devices:
- Flexibility of software and maps. Just about any digital map you can find can be used for navigation.
- Can be used as a PDA to store useful information on the road
Disadvantages:
- Not waterproof, but you can get bulky waterproof enclosures from companies like Otter
- Poor screens in sunlight
- Short battery life (
- No 12 V input, so somewhat more complicated to install if not running from internal battery
- Less stable operating system (possibly due to 3rd party applications)
- More fragile
For boating, I still go with a PDA because it is most important to be able to load up to date charts from wherever I can find them. For on-road touring, Garmin has won me over. But if I were going off road, I'd want something more flexibile than the 2820 or any of the devices that focus on turn-by-turn navigation.
The mapping data used in the Garmin units all comes from Navteq. The mapping data in the TomTom units comes from TeleAtlas. Between them, Navteq and TeleAtlas control the market in mapping data and do a pretty good job of keeping each other on their toes. Not much to distinguish them on that score, in my experience.
One thing I would definitely recommend against: a PDA with a Bluetooth GPS receiver. Not only are these much more expensive, but you'll feel like an idiot in tangled cables when you have to plug the PDA and the "wireless" receiver into a Y-adapter to the cigarette lighter to keep them going for more than a few hours.
sws