I bought my TomTom app a few hours after it came out. I have since used it at least 5 times...twice for my 150 mile round trip to school (no joke), twice in the city, and once to test it out initially the night of.
My drive to and from school is all back country roads and both times, I never had an issue. It kept me on lock and when I didn't want to turn where it told me to, it re-routed me in about 2-3 seconds.
The times that I've used it in the city, I began having signal lock problems. It only seemed to occur in areas of congestion (traffic) and also in areas where there were many roads around you (large intersections along the interstate highway). Otherwise I never had a problem.
I will note that the GPS when using the Google Maps app does pin my location correctly and reflects the same behavior as mentioned above (city and school routes). I will also note that in Google Maps, when you initially pin your location, it finds you, then zooms in, it locates me accurately with JUST the blue dot.....however, if I zoom in even once more than the default zoom, it shows a blue circle around my location which does not resolve no matter where I am or the circumstances.
It never did this with my 3G running pre 3.0...just my 3GS. It also never did this while running pre 3.0 software....just on 3.0 only (same GPS results when I upgraded my 3G to 3.0 as well).