If you've already got a Bluetooth, SDIO or CF enabled PDA, you may want to consider a Bluetooth GPS "puck" and software such as iGuidance. Turn-by-turn directions, POI (points of interest such as restaurants, gas stations, museums, parking, etc), storable "favorites," and pretty much every feature available in the dedicated units.

Bundles with the receiver and software are well under $200 . . . the Bluetooth receivers are about the size of a "C" cell battery.

I use iGuidance 3 on my Dell Axim X51v along with a GlobalSat BT338. Satellite acquisition is in the 15-30 second range, route calculations are reasonably fast, and the POI database is quite good. You can also customize the map sets loaded onto the PDA to conserve memory (I bought a 4GB CF card for my PDA an loaded everything on it, though).