I have both Waze (free GPS) and Navigon ($60 for North America + Traffic), and I find myself using Waze more often now because (1) it does better traffic-based routing and traffic-based time estimates, and (2) it works even when you don't have an exact GPS lock (e.g. inside a parking garage), while Navigon needs an exact GPS lock or else it will refuse to give you any route. However, I do keep Navigon around because it has (1) more reliable maps, (the Waze maps are all user-contributed so there are sometimes errors, like highway exits being marked with the wrong number), (2) it claims it can work without a data connection, and (3) it has nice graphics for highway exits that are much more useful than what Waze gives you when you get off a highway. Sometimes I'll even have both Navigon and Waze running at the same time now.