So for 50$ what does it do that Google navigation does or does not do? Better or worse?
TomTom downloads the maps to your device. That means no data connection is needed.
That was important to me - the ability to use 3G was broken in my iPhone 3Gs. TomTom fixed it. Google Maps was a no-show, by the time it downloaded a map, I had driven past the area.
My iPhone could still connect using Edge, so I was able to take advantage of the traffic data that TomTom and others provide.
Also, I have found TomTom to consistently be better at routing than Google Maps or others such as Sygic. You pay more, but you seem to get a much better product. When it says you will arrive at 4:16PM, you will arrive then unless you make stops or speed - it's accuracy was amazing.
I also found the user interface of TomTom on my iPhone to be better than the others I tried. That can be a personal preference, but it seemed to provide more information that was easier to read than the others I tried.
Turn by turn using street names was useful. TomTom also shows the name of the next street at the top of the display, so you can see it before you get to it.
When driving through Montreal and Quebec City there is a huge amount of hightway construction. It will be impressive once complete, but for now the roads are moved and many off ramps are closed. TomTom did not know about all of these changes, but it was able to get us where we were going easily by quickly recalculating as we passed each no-turn point. It did so as well as keeping us out of the major traffic jams as much as possible. Nothing that other navigation apps can't do, but it did do this very well.
The only drawback that we found was that TomTom did not know about the street that the casino in Moncton was on! Google Maps did know, so we used it for the last bit of navigation on my girlfriend's iPhone. Other than that, it has been brillaint for us, well worth the cost IMHO.