The iPhone 4 had problems from the day it came out from what I heard. It has to do with the wrap around antenna and the way you hold it. It drops calls left and right if you do not hold it precisely right. BUT, you can pick up a quick-fix for it .... for only another $30 ... unbelievable~
If you decide to get one, good luck with it. That was the only problem I heard about. I did hear that a FREE solution to the problem is to take an elastic band, like women wear in their hair for pony tails, and stretch it around the antenna. Kind of dorky looking though I think.