It works pretty well, BigCiX. The only complaint I have concerns the low volume of the voice commands. The developers are supposed to be addressing this issue in a future update, though I'm not sure if it's really a software problem or a limitation of the iPhone's hardware.
I tried it out yesterday and I'm glad I didn't have to pay for it. It works ok since it's free but it is very jumpy, it's far from being smooth like the native map app. It seems a bit buggy as well, I would get the "wrong way" msg at random times and the speedometer is also off as well.
I tried it yesterday on a drive into Vegas. It was "OK" for a free app. But the "Wrong Way!" crap was quite annoying. The there's the fact I drove most of the way with KPH instead of MPH, nor realizing it was a simple settings change.