My Dad has an iPhone 5, while I'm "stuck" with a 4S until the next iPhone is released. The iPhone 5 is a really nice upgrade if you want more rows of information on the screen.
But over the last two weeks, my Dad has been complaining to anyone who would listen about coverage issues. Often he would have two bars or less of cellular signal. He even went as so far to place several calls with AT&T support and even a trip to the Apple Store Genius Bar.
Turns out that there may not have been any problems with his phone. The real cause may really be poor LTE coverage. The fix for my Dad's coverage problem was to turn off the LTE radio by setting the Enable LTE option to "Off".