Since it's time for people to start seriously comparing GPS issues, I should point out that iPhone OS 3.0 has some kind of bug that harms GPS accuracy and slows it down (sometimes a lot) for some people. Apple has acknowledged the problem (at least to me!) so I know I'm not crazy.
But that means if I got Navigon or Tom-Tom right now, until Apple releases a patch, I'd suffer poor performance. This doesn't happen to all 3.0 phones by any means, but it happens to some, so keep it in mind as people review GPS software: accuracy issues should be laid at the doorstep of Location Services, not the GPS app.
Anyway, my 3G has had that issue but it has improved lately. Once 3.1 fixes it for good, I can resume use of my iPhone as my driving GPS. I've had my iPhone dash-mounted for a year, and it's great! For music--Pandora especially--and for GPS using Maps. Add a REAL GPS app and I'll be in heaven. I don't feel the need to throw money at a special kit--I already have a charger, and my phone is already mounted (home-made but effective) high enough to get a good clean GPS signal that I have relied on for a year while driving. I just want turn-by-turn guidance and a map dataset that doesn't depend on a data link. Once I delete some music
Right now, I'm still leaning toward Navigon. Those 3D interchange views look really helpful.
As for price, you're mainly paying for the software and the map data, not the little hardware piece. I don't object to an app that costs almost as much as a standalone. A standalone GPS would be WORSE--I'd have to lug it around or have it stolen. And would I have it with me in other cars when I wanted it? Usually not. All-in-one is the way to go for me.
You can do that with Navigon.
If you receive a call while using Navigon, all you have to do it relaunch Navigon and continue as before. You are able to talk and run the maps at the same time.
And the best thing about iPhone OS 3.0 is that remote-controlling your iPod NO LONGER brings up the iPod app! I can be driving, using Maps (or a GPS app), and still control my music by the remote control built into my charger. It used to be that I'd skip a song or pause it, and the map would always vanish as the iPod came up. The way it works in 3.0 is much nicer.