I am with Sprint and have investigated the switch, the coverage in most populated areas,and along interstates is good. My sources say it is similar to what Sprint used to be some years ago.
AT&T, Verizon have slightly better coverage. The great feature about GSM is the word wide roaming feature, (changing the SIMM card) the rates vary depending where you roam (developed countries vs. non developed) Europe is great. AT&T sells a GSM, but I have been told it is locked to their system, and they are not too willing to unlock. T-Mobile will sell theirs unlocked, so if you are not happy you can change providers.