I have an HP iPAQ with GPS antenna running Windows Mobile and I have installed TomTom.
The problem is the maps. We couldn't use it in remote places or Asia as at the time there were no maps available.
But the phone itself was very useful, serving as trip log, journal, WiFi for email and also GS-911 diagnostic via Bluetooth.
It lasted well and I'm still using it although the battery died and I had to get it replaced and I don't think I used it more than a couple of times for GPS as the application was too power thirsty for the battery...
You have to ask yourself, if it fails, will you lose all your data. If so then it's not good, we ran the risk and it bit us twice but it certainly saved on a lot of space as for us it replaced the need for a phone, gps and computer.