Pros: Decent phones, (usually) good service, random, occasional perks
Cons: No service in some areas, staff at the store where we purchased everything can be...iffy.
Summary: I begged my parents to switch from Verizon to T-Mobile because I wanted a Sidekick. We got a pretty good deal for switching - since there were five of us they dropped the $35 activation fee. Everyone was really nice the first few times we went in there, and when my sister had a problem with her Sidekick data plan they were very pleasant and fixed the problem. Also, we got a ten dollar off credit for the bill after our one year anniversary.
Now, that being said, I do have to say I was a little disappointed going back to the store when my mom had a problem with her phone. While the problem was resolved, it took some time, and she had to get a new phone shipped to us. She had to use a phone from them while she waited for her new one. Why she had to pay for the shipping or replacement or something for thee new phone is kind of beyond me, but I'm not T-Mobile (it was only $10 but still). Also, the staff at the store became kind of...rude after the sale. I'm sure it's that way with the majority of the cell phone providers, but it just kinda irked me. Going from super nice to I-could-really-care-less after a huge sale? Really?
All in all, I love T-Mobile. The have some good phones, and reception is usually good (I don't get service everywhere, but at least where I need it for the most part).