A buddy of mine has the Magellan PDA-based system. Seems to mostly work, although they aren't as easy to use as the built-ins, plus having this thing on the dashboard just screams "steal me". Also, the built-ins combine GPS with wheel sensor data, so even if they can't find the satellite, they can still do dead reckoning for a while, e.g., in an underpass or in a downtown area with tall buildings. And, you actually do get a software upgrade when you buy new map data. It's all on the same DVD. Likewise, with the 8" touch screen and voice recognition, it seems like they did a better job on human factors than you could ever do on a PDA gizmo.

On the flip side, every time I'm in a rental car in a strange city, a PDA gizmo would be a big improvement over nothing...