Coverage and the largest 4G network available in the world.
Speeds aren't the fastest (AT&T actually has that but they don't have nearly as much 4G cpvrrage).
For one line I honestly don't think that it's worth it unless you travel a lot. Then you'd need the coverage. Especially considering that you're giving up unlimited data, unmodified phones, delayed phones, and super later than they should be updates to go there.
If you have 2 or more lines its more than worth it for the coverage.
sent from the best smart phone (not phablet) on the worst network- the galaxy S III unfortunately on T-Mobile