Yup, I got myself an iPhone 4 to replace my 3GS when they came out and had no end of problems, but still (for some reason) insist on using one!
On the first one the screen was broken (a common complaint on first releases)
Second: The home button wouldn't work unless you pushed it stupidly hard
Third (my current one) is starting to develop the same home button problem...
On my older iPhone's it was mainly the vibrator switch that went insane every time you shook the phone
Kudos to the Apple store though, they just give you a new one every time!
The main thing about the iPhone 4 is the screen, it IS
a whole lot better and it makes reading websites/text a considerable amount easier. As well as that the gyro (new to the iPhone 4) is actually quite cool to use in games!
Apart from that, we all know the signal issues, but they're fixing that in the next release with more insulation apparently...
Flown On: A319/A320/A321/A332/A333/AT45/734/736/738/744/DH8D/T204/T154/IL62/T134/IL-18/An-24