In lab tests, Consumer Reports says the signal "can significantly degrade to cause you to lose your connection altogether if you're in an area with a weak signal." Three iPhone 4s were tested in an RF isolation chamber, and while the signal fell out on the new iPhone, CR didn't see the same issues on an iPhone 3GS or a Palm Pre.
The tests also showed that putting some duct tape (of course!!) on the lower left portion of the iPhone antenna strip helps a great deal, and they speculate a case would do the trick as well. The testing firm adds that the iPhone 4 scored at the top of the list in every other feature, so the reception issue is a deal-killer for the firm. CR says that Apple will have to provide "a permanent -- and free fix" before it recommends the iPhone 4.
Apple iPhone 6s
Apple iPhone 4