I think T-Mobile's FUP is 1GB not 3GB (Apologies, I stand corrected, it is indeed 3GB for Android phones only). For me personally I have wifi at home, and based on a fair bit of browsing outside, twitter updates and some sat nav usage, my usage app (3G Watchdog - Heartily recommend) tells me I am likely to use only about 15% of that. The surprise to me was the sat nav use, which appears to be minimal at best assuming you don't overlay the map with satellite view (Drove 30 miles using it and used only just over 1MB).
As for the deal on the phone, I was 9 months past my contract end date, and my usage is not that much, so when I called to cancel my contract, they went away and came back with