We've been using Navigon for iOs extensively during our latest US/Canada road trip.
It does well in rural areas, but in urban areas we've twice run into significant glitches. On two occasions, just short of a key junction, Navigon abruptly changed its recommendation.
Navigon acted as though its lost track of where we are, and it transiently thinks we've missed a turnoff. We get the recommendation for backtracking -- before we go off track.
It's like getting a message from the future; a self-fulfilling message because the updated directions make us go off-track (so causality is not violated). This can be disturbing, as when Navigon sent us for an unplanned tour of Detroit.
I assume this is a GPS satellite access problem, perhaps exacerbated by a lack of useful wifi data. I've not seen this discussed much however. I'd like to know, for example, dedicated devices have less problems than iOS devices. Do some software packages handle this better than others? Are there ways to minimize glitches? (Example: try not to vary speed as one approaches key junctions).
I'm quite fond of my 4S/Navigon package and the car windshield mount we use, but I could do without the unexpected tours. At least I've gotten enough experience that when the GPS goes haywire I'm less prone to make a sudden, dangerous, lane change.