Coverage? is an iOS app that tells you where to expect coverage from all the major US carriers. The maps are based on coverage maps from the cell companies, but they are interpreted, updated, and quite a bit more useful. You can see all major US carriers, and overlay them. I initially looked at Coverage? in 2010 when the iPhone was only on AT&T. At the time, I found the comparisons a bit depressing. The Verizon coverage was pervasive, and the AT&T coverage was pretty sparse outside the big cities.
Fast forward to today, and Coverage? is out with a new version that has updated maps, and, for the first time, maps that include AT&T LTE signals. The current iPhone and iPad don't work on LTE, but I suspect that is about to change. Also new is a search feature so you can get the good or bad news for places you plan to visit.
I found the original version very useful when I was on the road, and in almost every case it confirmed why my signal was good or bad. There are also interesting nuggets that can be gleaned from comparing old and new maps. If you check the gallery comparing maps from early 2011 and 2012, you'll see Sprint has almost completely vanished from Montana and Wyoming while AT&T has moved in.
Coverage? is a free update to current users, and US $1.99 if you're buying it for the first time. It's on sale for 99 cents through the weekend. Maps cover the entire continental USA, Alaska, Hawaii, Puerto Rico, and the US Virgin Islands for AT&T, Sprint, Verizon & T-Mobile. 'Coverage?' is not intended to give city level details - it's goal is to provide an overview while traveling. You can catch a video of the app working to see if it will be useful to you. I like Coverage?. If you travel I think you will too.