Unlimited 2G data. LTE is limited to 5GB per month. I've hit their throttle before, and while it's nice that you don't go dark, it's not for normal use.
It depends. 1.5 hours of facetime will use ~1gb, and streaming 1 hour of music every day will run at 1gb over the month. Add browsing during commute and you're getting close. It's probably not "normal" in the sense of average, but for the heavy consumer, they will have to moderate their usage.
I'm on the verge of doing the exact same thing. On that plan are you able to use any of that data for tethering or mobile hotspot? A customer service rep said you can use all of your "up to 5GB" 4G data for tethering if you want, but I found that a little hard to believe.
http://www.t-mobile.com/cell-phone-plans/individual.html