First, they are a T-Mobile MVNO, so whether or not T has good service in your area is key. You can check what T-Mobile claims at their website. There are also apps for your phone like Cell Mapper or Open Signal, that show what other people who loaded that app and ran a test are actually getting. They show dots for each spot a test was run and how good it was. Also it's GSM, so your phone needs to be compatible with T-Mobile, have a SIM card slot.
Mint Sim is a sub of Ultra Mobile, which is a T-Mobile MVNO. So the deal is you get unlimited voice, texts and data for:
$15 month 2GB $20 5GB $25 10GB
If you pay for a year of service in advance. The rates are higher if you buy just 3 or 6 months at a time. But right now they have a deal going where if you buy any of the three month trial plans, you get it at the yearly rate *and* they give you 6 months instead of three. So, I just got 6 months worth of service with 10GB of data a month for $75. After that, it will be $25 a month. And those trial offers, you have 7 days from activation to bail out and get a refund.
I've previously been on the Sprint network and let me tell you, T-Mobile blows it away, easily two to three times as fast and far stronger coverage. I was testing last night at home and getting 76 Mbit/sec down.
The downside is they are a sub of a sub. Sounds like their support is in India. From what I've seen, people have either trouble getting it to work or it works and it works really great. If you have trouble people have said they have trouble getting to a live person, have to use email, etc. But I called up and got through to someone in about 3 minutes.
Final thing, they also have a new $5 trial deal, with two sim cards. The first sim card is good for 100 mins of voice, 100 txts, 100mb of data, so you can try it out. If you like it, then you go online, pay for one of the plans using the second sim card. They apply your $5 to the new plan too. I think the two for one deal can be used too.
I've had it 3 days now, working solid. Just driving around here in NJ I have 3 or 4 signal bars almost all the time, while with the Sprint network I had 2 or 3. Call quality is excellent, better too.