I use T-Mobile.. now I understand the rules so when it works it works great.. Alas, the key word is When.. 4G coverage is not as good as I'd like but usually I can get either park Wi-Fi (in use now) or 4G (this morning at a different park) so I am happy with it.
My account limit is 6.5 gig of High Speed, then it drops to 2G.. I have gone over twice in the last year, Usually by about 0.1 Gig. And on the last day of the billing period. Overage charges are.. Not on the T-Mobile plan I have.. They just throttle you.
It is important to know how much bandwidth you need, Drop into McDonalds or the local library for a bit if you must to extend your limit. I do keep a watch on mine.