This is a new app, so it doesn't have much data associated with it yet, but with a large enough user base, it could potentially be very useful for new users. The app is associated with a website, http://carriercoverage.com, which currently only shows 1764 users and 77 devices, but as those numbers grow, so will the data base. It's developed by the same company that owns phandroid.com and you can see their review here: http://phandroid.com/2012/02/06/carrier-coverage-launches-on-android-market/ so take that for what it's worth. They are open about what data they collect and how they use it (no selling your data). With the recent CarrierIQ scare recently, some may not want to go for something like this, but I think this one is legit. Unfortunately, data from rooted users is not included because that enables you to edit information that is vital to the data points they are looking for (carrier and device), but that won't apply to most of you. Let's expand the user base here, and see what this thing is really capable of.
Personally I use Sensorly for my coverage map comparisons.
I was using that app for a while as well. It seems to have a decent user base, the only problem is, it only gives readings for the location of the device (you can see the coverage areas follow the roadways). I'm hoping this app will give a little broader signal strength layout.