I just got back from Arizona where I viewed the Grand Canyon, visited folks in Phoenix and headed back home to Los Angeles using nothing but my iPhone and it's native Maps application.
Color me surprised, but I had 4 bars 99.9% of the time while in the California and Arizona deserts. The only time I lost reception was in a house while visiting a small town called Chino Valley, and even then I picked up on someone's wifi and was still able to browse.
Battery life was very good, it included listening to a couple albums and watching a few 30 minute episodes. I made calls also, but they weren't very long, more like a few minutes just to let people know we were on our way.
As far as maps are concerned they were all very accurate and up to date. By reading the road signs we were passing, I found it relatively easy to pinpoint our location on the map. The only hiccup in the process was EDGE loading new map tiles, other than that it was dead simple. One cool thing about finding your location on the map is making that your new starting point and having the Map Application tally the miles that are remaining to reach your destination.
On the way back we ran in to traffic, we had accurate reports whilst in metropolitan cities, however the stretch from outside Phoenix to Redlands wasn't covered. It was a shame, because there was a ton of traffic from Palm Springs to Redlands.
Lastly, we took a GPS with us just to be safe, and I'm happy to say it was never turned on the entire trip
I've always found my iPhone to be a great device, but now I find it to also be a great value. It wasn't cheap, and I've waited a long time for this phone to prove it's worth, I'm glad it accomplished that feat, most electronics never do.
Pics from my iPhone
edit - I had no idea iPhoto interpolates the iPhone photos. They never seemed that large when xferring them from my PC.