Just got back from a long road trip from Texas to Arkansas, Tennessee, North Carolina, Kentucky, Illinois and back. Had a couple of instances where the GPS receiver didn't seem to get a fix but reactivating seemed to take care of those. There was a very long stretch of I65 between the Ft Knox and Bowling Green areas where we were roaming and didn't have Verizon data so the Droids weren't of much use. Overall though, despite the frequent posts I see from people recommending stand alone GPS units, I was very happy with the Droid. However, my nav needs are fairly simple. The last road trip we took was 5,000+ miles and we only relied on Telenav running on a Blackberry 8310. No complaints there either.
Also, it's quite pricey (IIRC $70 plus you need a compatible vehicle mount) but I highly recommend ProClip's dock for the Droid. Made (secure) mounting and charging our Droids very easy.