Since Mint Mobile runs on the T-Mobile network, performance is how I expected T-Mobile to be in my area. Maybe not as good as Verizon or AT&T but completely acceptable for my uses especially considering the savings. I think it will really depend on the coverage map for your area and usage. I tend to bounce between destinations that all have WiFi (work, home, etc.) so I don't seem to use a ton of data. Just set your data hungry apps to only download on WiFi and good to go. If you are someone that relies on your carrier for work and in a rural area or dead zone, it might not be for you. Of course, there are MVNOs for AT&T and Verizon but they cost more per month than Mint.
I will say it feels very nice to pay about $200 up front and then not to have to worry about a phone bill for another year.