Their coverage maps offer a good indication as to what to expect in a general area, but they lack the precision necessary to perform due diligence before making a switch.

I work in Manhattan, where I've twice had the misfortune of occupying a cubicle in the exact center of a building. Both T-Mobile and AT&T report full coverage for the entire area, but the many micro dead spots in homes and offices can be a real problem. T-Mobile in particular licenses spectrum that can't penetrate walls as well as some other parts of the spectrum.

I'm sure there's a startup for providing user sourced coverage data at a very granular level, though it would be difficult to deal with the constant changes the carriers are making to their networks.