I'm gonna break it down for everyone by carrier. Agree, disagree whatever.
Metro : This shit is for the people who can't consistently pay a bill on time, have super bad credit, or a teenager that never goes anywhere but school and home. Don't get metro if you need a phone for work, or travel. You will be left stranded since the network is horrible.
T-Mobile: The plans are cool, decent phone selection but the network is not that great. Every time I really needed my phone to work when I had T-Mo, whether I got a flat, or tried calling to my job, I had weak service. If you are willing to sacrifice service for price, than T-Mo is a good option. Not great, but not the worst either.
Sprint: Sprint has a good network, and a nice selection of phones. The unlimited plans are very attractive as well. The cool thing about sprint is that the prices keep dropping, but it doesn't seem to be helping much. Customer service for sprint is out of this world retarded and the churn numbers prove it. These guys lose a million customers per quarter.
Att: The voice network is cool, but the 3g data network is wack as fuck, and that's because here in Nor Cal it's better than most places. Those with data devices can tell you. Don't lie, shit is garbage. They have the best selection of smart phones and the Iphone. Price is equal to Verizons
Verizon: Verizon has the best network hands down. It's not as fast as Sprints, but the coverage is 10x better than sprints and ATT's. One example, I was able to stream Sactown Radio on my PDA over to my stereo via bluetooth from Hollister to San Jose without a hitch, never got any interruptions at all. Verizon has the shittiest selection of phones but that is starting to change with more handsets launching faster than before. The downside is price. It's typically the same as ATT but still more expensive than what people get around the world for half the cost. If you really need reliability and have a few more bucks, verizon is the way to go.
With all that said, like any service you will have to sacrifice something, to get another thing. Cost of service for ....Network Coverage etc.