I use T-Mobile, primarily for one reason: they were the least extortionate in what I needed to pay them to compensate for the fact I didn't have a credit rating (why is that even needed for a bloody mobile?).
They have however managed to get another major advantage (for me) since then, which is FREE international data roaming. It's only 2G speed, but I love being able to check my e-mail and FB and receive WhatsApps etc. (for all of which 2G is easily fast enough) when I'm visiting my family back home.
On the downside, their reception sucks in a lot of places.
Back in the Netherlands, I had a monthly contract (no fixed duration, could cancel any time) with 1GB of data for €12 per month. In the UK I had a prepaid plan with 1GB of data for 10GBP per month. WHY IS IT SO RIDICULOUSLY EXPENSIVE HERE IN THE US!?