I haven't noticed a compatibility issue with my devices yet (I have a car kit and a TomTom kit on it's way to me). Many times my 3G would give me an incompatibility warning when using my car kit, which I just told it to ignore, but I haven't seen it on the 4G yet. As for the signal issue - I did experiment with it and noticed that the bars did drop after holding it in the palm of my left hand, you have to have the bottom left corner of the device in the palm of your hand, with your palm making contact with two of the antennas to make the signal drop, but it definitely drops, which is a bad thing. Isn't an issue for me as I live in California and have to use a bluetooth anyways, and secondly because I am right handed and swear that I am going deaf in my left ear, so the device is always in my right hand while using it.
There is another issue that has popped up, and I noticed it on mine when I got it. There are small spots of yellow on the screen, but they go away after using the phone for a day. It turns out that the adhesive Apple(Foxconn China) uses to adhere the panels of glass together didn't have enough time from manufacture to delivery to allow the adhesive the correct amount of time to evaporate. Once the phone warmed up and the evaporation completed the yellowing goes away completely.