I probably use the GPS on my phone more than anything else, including calls. I have about 6 programs that access the GPS, one for driving, one for being out in the boondocks, one for being able to tell what stars are up there at any time and any place, and even more stuff.
Semsons has excellent "open box" BT receivers at really good prices -
http://www.semsons.com/bluetoothgps.html. Check it out. I have a GlobalSat BT-338 and use iGuidance as my regular driving software. It makes driving totally stress free, even when I'm in a rental in a strange city - the screen really is big enough to see the maps, and the voice commands make it super easy.
I haven't had any problems using the BT receiver and a BT headset at the same time.
Spend a little time over at the forums at gpspassion - you'll learn more than you ever imagined. - sheureka