I saw some inaccurate data when I first reviewed Waze, and I'm still seeing it. Since everything in Waze needs to be downloaded, it depends on a good internet connection to work well. I also saw problems from the Waze servers: when I tried to calculate a route, I got a server timeout error.
I tried Waze on a long trip, and there was no real traffic info of use when I was away from cities, not to mention that when I lost 3G signal my navigating came to an end.
At the end of the day, liking this app depends on what you want in a navigation program. Personally, I want to put in a destination and get there quickly and efficiently. You can use Waze that way, but it's designed to let you participate. Your rate of travel is added automatically, but you can also upload pictures of traffic incidents and send text about delays. Happily, Waze won't let you do that while you are moving, but it's just too much fiddling around in the car for me.
If you really crave a free nav app, by all means take a look at Waze. In a city with a lot of drivers using the app, the traffic alerts can be first rate. On the other hand, the paid nav apps keep dropping in price, have much more accurate street data, and generally more features, so they are also worthy of consideration.
Waze works on any iPhone with a GPS, and there is also an iPad version. The iPhone version does not run in the background in iOS 4 just yet, so be warned about that.
[Note: Waze crashed when an email came in and would then not work properly at all in the background. Some of our readers insisted it did run fine for them, so I did a re-boot of my iPhone and took Waze out for another quick drive. This time it did run properly in the background and I could hear voice directions. I'm not happy that the app crashed but happy to see it does continue to function after a fresh startup. Thanks to our readers who urged me to give it another try.]