sbiddle: T-Mobile is terrible in the US. ajobbins have me his SIM card to use last year after he visited but I don't recall him warning me how bad the experience would actually be!
It's generally dependant on the area you're in as to which provider has the best service quality. Verizon tends to be the most consistent across the country, but is more expensive and obviously not as accessible to international travellers.
Anyway, as a 2Degrees mobile customer I'm a bit jealous.