I've been using a PDA-based Sat Nav for a few years now. Overall usefulness is great (it syncs with my Outlook calender, has my contacts on it, MP3 player, etc); but if you're looking at something like a nuvi 660, go with the 660.
Bigger screen, more useful navigation-related software, TMC traffic updates; and in this case, you can use it as a Bluetooth hands-free kit, too! Which is portable from car-to-car
So, if your primary focus is actually the Sat Nav element, that's the way to go. If you're only going to use that functionality once in a while, only then is the PDA-route is worth considering.
In my humble opinion, of course! (which is why I'm now looking to get a nuvi myself)