I've already tested Navigon a little bit, and it works well. The only issue is its massive drain on the battery, especially when listening to music on the iPod while driving. But this will be addressed by a phone stand charger. I've seen two interesting models so far from Belkin and Griffin. So, even the announced, but as yet unavailable TomTom stand is no argument.
Well, I've taken the jump at last and bought Navigon from the App Store. My trusty old TomTom GO 700 will be gone by tomorrow, and so will the special price for Navigon (hurry if you're planning to do the same). But that's not the only reason: I've been waiting for the TomTom app to come out because it is the market leader, but reading comparative reviews, the two competitors seem to be running side by side, with a slight advantage for Navigon. For me, the main reason for choosing Navigon over TomTom is that their coverage is substantially better (more countries in Eastern Europe). The substantially better price (for now) is rather convincing, too.