To the OP, as mentioned before, it really depends on where you are. You could always test out T-Mobile's service easily by using Test Drive offer. They send you an iPhone 5s for you to use for a week, for free. At the end, you return the phone to any T-Mobile store. Unfortunately, you can't do this with any of their phones--only the iPhone. But it's a great way to test out the network.
If you do have a GSM-compatible phone, you could also get a SIM card from T-Mobile and try it out for as long as you like, since they don't have long-term contracts. If you don't like it, stop it at any time without an early termination fee.