I just drove several miles with TomTom tonight, not really enough to give it a fair review. Though on first inspection, I do prefer Navigon's 3D view, especially having the ability to see further ahead. It seems to show more depth of field than TomTom.
Though I've yet to drive on the freeway with TomTom, I'm told it doesn't name the freeway exits like Navigon. (e.g. "Exit i405 North in...") This is a very useful feature that would be missed by many.
I also missed the POI icons that Navigon offers for individual stores, gas stations, etc. It's a nice touch. I also like the dark, simple look of Navigon's menu, though I'm not sure which I prefer. And the iPod issue that MBHockey mentioned (pausing audio), which I haven't checked, is another BIG drawback for me.
I'll test it out a few more days and post again. One thing's for sure, TomRom's definitely not much better than Navigon, if at all.
The three big things TomTom has going for it IMO on the Maps is that they are newer, have more color options, and the text is much more legible on street names. Also, I can't explain it but they just look cleaner but I'm not sure why.
Really the thing that is the biggest difference in Navigon and TomTom is the routing options. TomTom has many more.
It still doesn't justify the cost though. They should be selling TomTom for $60 max, $80 for the cradle alone (for people who want to upgrade later), $120 for the bundle. Also, the cradle should be usable with cases (too bad it probably won't judging from pictures) and they should add TTS as soon as possible.
Navigon is overpriced too though. It should be like $50 considering its smaller feature set.