I've been tinkering a lot with Google Maps lately on my Windows Mobile based phone - and it just doesn't quite have the edge to dislodge TomTom as a navigation platform. There are multiple problems with the way Google Maps is implemented that takes it from navigation solution to a mere playtoy.
1) Route planning. If you've ever had much of a play with the Get Directions part of Google Maps, then you know what I mean by this. Google Maps is known to not do this task very well. As an example of this, take a look at this screenshot of it's awesome route planning skills :)
2) No adaptive route planning. Whenever you drive off the recommended route, the planning doesn't keep up - this means if a road is closed, or you ignore a stupid plan like the one as an example to point #1 then you are out of luck in having Google Maps re-plan the route for you. Sure, you can manually get it to re-plot things from your current location and start the whole game again, but this is a trivial matter that should be handled automatically!
3) Stupid backlight handling! This is a major point for me. While Google have made a good point and made the software respect the backlight settings that Windows Mobile have been set with (via Start -> Settings -> System -> Backlight), the most useful purpose of Google Maps is rendered useless by having to keep tapping the screen or a button to stop the backlight turning off while attempting to use Google Maps as navigation software!
If these above issues can be fixed in newer versions of Google Maps then TomTom may have some very good competition - as the integration of searching for businesses etc within Google Maps would be VERY hard for TomTom to compete with - but at the moment, I won't be switching my navigation software to Google Maps any time soon.