Mapquest has a pretty solid iPhone app that provides voice-guided navigation for FREE. I have always HATED Mapquest's setup on desktop machines, but I am quite impressed by the iPhone app, especially for the price. It's really easy to use and has a lot of great in-app features like Live Traffic that updates every 5 minutes as well as very easy ways to show anything from bars and restuarants to airports and gas stations. It also has Auto Re-Route, which is a GREAT feature that is missing from Google Maps. Like I said, I have ALWAYS been a Google Maps fan, but when I go somewhere that I am unfamilar with or am driving in bad traffic, I don't hesitate to open up Mapquest instead of Google.
One warning: Using the Mapquest app tends to use a lot of data. I downgraded my data plan from unlimited to 2GB. Last month I used the MQ app for a couple long road trips and a few days later got an email saying that I was nearing the limit of my data plan (although I continued to use my phone like normal and never went over the 2GB, so the "heads-up" email is sent LONG before you actually run out of data). Not THAT big of a deal, but just a heads up. AT&T only charges $10 when you exceed the 2GB though and that gives you another whole GB to use, which is WAY cheaper than purchasing many of the other Navi apps.
Long story short: This app is free and it WORKS. I've yet to have any real problems with it. Yes it could do some things better, but for FREE voice-guided nav with a TON of built-in features, this is a no-brainer. Download, give it a shot, you may be suprised by how useful it is and how well it works.
(and NO, I DO NOT work for Mapquest. Like I said, until I tried this app I HATED them. Things have changed, that's for sure.)
Enjoy