You're from MA so this should apply to you.

-Cingular = crap avoid
-T-Mobile = it has super good service some places, lousy others. Its all up to luck. it uses GSM so you can use any phone really. Its the best bang for the buck as far as minutes goes, but their customer service is kinda assh*ly.
-AT&T = pretty good service. Expensive, also uses GSM
-Verizon = No first hand experience but my friend has it and its awesome, he can get calls in many places I cant. Huge downside is they dont use GSM so you gotta get a Verizon only phone
-Nextel = No clue