I agree with dictoresno. Although I've never used it on a smart phone, you can't usually go wrong with Garmin products. I have been using Telenav for about 3 years and it's also very good but costs more than Garmin. It's paid for by the company I work for or I would use Garmin. Not because it's better, just because of cost.
You can fuss about with some free GPS apps that aren't bad but IMO if you want/need something that is pretty reliable, Garmin and Telenav are your two only good choices.
I agree that Telenav is decent, not great. Every phone app I've seen w/ turn-by-turn is slower to update than a dedicated gps device. Speed is sufficient for suburban or rural areas, but in an urban area like downtown Chicago it gets buggy and becomes pretty useless. But in fairness, this may be more a result of signal loss from tall buildings than speed alone. Dedicated GPS devices also struggle to keep a signal lock in these areas.
In terms of update speed, a phone gps does fine until you get to an area where multiple turns are close together. The device is slower to respond than a dedicated gps, and at times you'll be past your turn before it updates.
It's convenient to have one device for phone and gps, but performance isn't quite as good as having a dedicated gps. You can buy a new Garmin Nuvii w/ lifetime map updates for around $100. I use ATT Navigator because my employer pays for it. If I had to pay I'd buy a stand alone GPS for most needs, and use Google Maps for the times I'm not in my car.
Garmin Mobile without a doubt. I have run this side by side to the dedicated unit in my personal vehicle (work vehicle does not have GPS) and it more than holds its own. It doesn't matter whether I am in the suburbs of CT or RI or in the larger cities it performs very very well.
I have used aMazeGPS, Nav4All and TeleNav and none of them come close to Garmin Mobile.
