I've used a lot of smartphone GPS solutions, and two dedicated units. The biggest issue with all smartphone solutions is that you cannot *rely* on them. I've used Navicore/Wayfinder, Nokia Maps and Google Maps on Droid. They make nice demos but seriously, you really want to buy a dedicated GPS unit. I paid recently $130 for a new unit on a sale, and it's far superior to any mobile stuff that is out there.
Just a quick comparison:
+ Dedicated GPS units are more robust. You can rely on them. Smartphone based solutions do crash. You cannot rely on them. That's the most important factor for dumping them for any serious usage. They lose signal without being able to restore it without reboot. Theylose data connection (only prob for Google Maps based navisystems, Nokia Maps is offline). They just stop working. Every smartphone based solution had problems (at least after using them some time, not in "demo sessions"...) and the dedicated units don't (crashes are very rare).
+ Dedicated GPS units have far superior signal strength compared to any smartphone based GPS. Just try it in a parking hall: Dedicated unit will pick a signal no prob, smartphone will not. Smartphone will also lose signal easily when you're driving where dedicated unit is not. Trust me, I've done lots of comparisons running dedicated units and phones side by side.
+ Dedicated units are much faster. Offline maps is the first key requirement where Google Maps based systems fail. I used Google Maps on Droid for couple of months (on T-Mobile) until time wasted restoring data connection, downloading map, getting signal, downloading map again, downloading instructions,
+ Dedicated units have better functionality and usability compared to any smartphone solution. For example, Google Maps does not have "search along route" functionality which is really nice during any road trips.
Still, I find uses for mobile GPS as well, but not as a dedicated GPS replacement but for "fun usage" e.g. during walking tour in a new city. Just don't buy mobile GPS solution and imagine that it's competing in the same league with dedicated units.
If you insist buying a mobile GPS, you can get Nokia 5230 with $160 or something like that. Nokia Ovi Maps can at least use offline maps, unlike Google Maps.