It uses an ATT network for my phone (which is what I had with a contract plan) at a low cost.
The plan I have now is unlimited voice/text and 3gb data per month. With taxes, the cost is $39 and change per month (automatic refill which gives you a $5 discount).
My present phone (and last previous one) are made by BLU - unlocked gsm) and I have been happy with them. Most of my data use is via wifi so does not count against the monthly limit.
People used to complain about Net10 customer service, but I have found it to be good. Setup my new phone sim porting my number all online through facebook customer service - maybe 20 minutes.
I haven't had to call them in over 4 years.
I have a "dumb" smart phone and pay $25/month for 1000 minutes.