I am one of the early adopter, having bought this kit early when it was first available. I travel by car quite a lot as part of my job and I had the chance to test this kit extensively.
As the title stated, here are the major problems:
1). CHARGING
When I plug my iPhone 3GS, about 3 out of 5 I'll get a message on the iPhone saying that "This accessory is not for iPhone". I have tried every possible way to ensure that I am plugging the iPhone properly, which it does because the shape of the kit prevent the iPhone to be plugged in other ways, yet I still get that dreaded message and the iPhone DOES NOT CHARGE. it seems that the connector is not designed well and it does not recognize the iPhone most of the time.
UPDATE: Here's another thing I found about the charging issue: the car kit is SUPPOSED to CHARGE your iPhone while you are using the naVigation apps (TomTom, Navigon, Magellan Roadmate) at close to high brightness (due to sunlight). But when you do this, the car kit WILL NOT CHARGE your iPhone. I have tested this with 2 iPhone 3GS and none of them charge when I use any GPS application. Since the Iphone is running continuously, this car kit adapter cannot keep up with the power need.
2). GPS
The GPS inside the kit, when it work, it worked great. But most of the time, it DOES NOT WORK. Many times, my Magellan GPS or Navigon app will suddenly lose signal. I thought it was the application itself, but when I mounted my iPhone to a normal car kit with no GPS, I never lose GPS signal unless I am inside a tunnel or under heavy foliage cover
I tried my best to test this car kit, even beyond the 30 days return policy. Alas, this kit just perform badly.
Do not buy this kit!