This app is excellent. I've used AT&T Navigator, and can say that Destinator beats it by a mile.
On a recent 6 hour trip using an iPhone 3G, destinator ran very smoothly even in 3d mode. While it is true that it takes awhile to load, the app is responsive and the routing was figured reasonably quickly.
In use, you can easily shut the iPhone down to save battery, then start it again when you need it. In addition, destinator comes with a maneuver mode that only shows the next direction and doesn't display the map. Destinator claims this saves battery life although I didn't test it since I had a car power supply cord.
Voice directions are good and the onscreen display will actually show you what the exit looks like. I did find a few odd routings where I was instructed to exit in the middle of downtown PHiladelphia as we passed through on the way from NJ to NY. This was obviously wrong and I ignored it.
In addition, the programmer crashed once after waking up from sleep. Relaunch fixed it and I had few problems with stability other Than that.
The interface is good but difficult to control in the car. The addresss lookup works fine buT I usually have to try twice to get it right. There's something that isn't quite right there that keeps it from being seamless. What would be awesome is to be able to input the address via voice recognition.
The iPod integration is a nice touch although expected due to the lack of multitasking on the iPhone.
All in all, this is a program that is easy to like and an excellent tool for navigation on th iPhone.