I've tried all, Garmin is actually really good and equivalent to both tomtom and navigon in quality. It could use some features that the others have (such as giving 3 route options) but all in all, it was worth the 50% off sale they had, which made it the cheapest full GPS app.
Can't compare it to TomTom as I only own Garmin and Navigon. So far I'm very impressed with Garmin 2.0 and it has become my favourite. I like the screen and layout, I like the fact that I can zoom the screen to a level I like and still maintain a heading up display and I like the fact that the speed limits are now showing on my display (Canada) for the first time ever. I also like the fact that Garmin provides a compass heading on the main nav screen.
I haven't had much of a chance to test it yet with an actual destination plugged in but one thing I do want to test is how "chatty" it will be. Navigon is great with the voice commands in that it provides lots of warning for impending turns, etc. In a lot of cases though Navigon was a little too chatty.
With Garmin 1.0 I found that the voice commands on occasion could be slow and sometimes were non-existent. There seemed to be no rhyme or reason to when it would issue or not issue a voice command. I'm hoping that Garmin 2.0 has addressed this and is somewhere between Navigon and Garmin 1.0 in "chattiness". Keep ya posted on that but so far I'm very impressed with Garmin as an overall GPS app. I like the search function much better than Navigon and when I do a search for let's say, gas stations, it brings up a list showing the distance AND a direction arrow for each listing showing me if the station is ahead of me, to the side or if I've already passed it.
I'm still waffling on paying for traffic as I'm not sure how well it will work. I did buy it for my Navigon but I found the traffic interface to be seriously unfriendly and frequently would not connect to the network for updated info resulting in a reload of the program to get it working again. Garmin wants $20 a year for theirs but it would be nice to see some feedback from others first on the user friendliness and accuracy of it before I fork out the bucks. I'm ok with $20 a year "if" it works as advertised.
That's all I can tell you so far but bottom line, I own Navigon and Garmin and I seem to now almost never use my Navigon. They are both using identical Navteq map sets so it really comes down to which interface you like and who has the best routing engine. In that regard YMMV. Cheers!
I purchased the new Garmin 2.0 GPS app after reading this thread. I have to admit that I really like it. The detail of the maps is very good, it even knows the names of the roads in a private arboretum that I frequent. I have not taken it on a road trip but I will in a couple of weeks for a long week-end.