I use Mapquest Navigator, and have for about 6 months. I love it! Yes, it occasionally has me turn on a dead end road, or takes me down a gravel road, but I live in rural Iowa. Can't expect it to be perfect every time. Here are the things that I really like about it.
1. Voice commands, with street names (I'm sure others have this as well, but I wouldn't have one that did not)
2. "Search Contacts". If you have entered addresses for your contacts, you can simply look up "Sam", and it will find all of the "Sams" in your contact list, and let you choose which one you want. Then it will get you directions to Sam's house.
3. Choose between "shortest route" and "fastest route"
4. Night mode, Turn mode, and Map mode. All are really great.
5. As someone else mentioned, it is fantastic at finding "places/businesses". I can type in "Golf", and it will find all of the golf courses, as well as any golf related businesses, in my area. Then if I highlight one of them, and hit the menu button, it offers me the option to call them
right from MQ Navigator!
This is awesome, in my opinion.
The only issue that I really have with it is that the "estimated travel time" is not usually very accurate. I usually get to my destination well before Navigator thought I should. But better early than late. And the screen also tells you how many miles you have to travel, so you can estimate it yourself better, probably.
The question was raised about battery life. I use Navigator in the "always on while navigating" mode, and it does drain the battery considerably faster. But like others have said, I just keep my phone plugged in, in the cradle, and it's no problem.
After 6 months of use, I am a firm believer in MQ Navigator. I will not be switching. It was well worth the money, in my opinion.