How's mint's coverage in the real world? I was checking out their map, but I never really trust those.

AFAICT, it is identical to what a T-Mobile subscriber would see, though I imagine as a Mint Mobile user, we are deprioritized compared to the current high-paying T-Mobile customers in a congested cell.

I use T-mobile now, so I suppose switching wouldn't be to much of an issue in terms of coverage. I'm currently grandfathered in on an old pay-as-you-go plan that gave 100 minutes a month (I don't talk on the phone much), unlimited texts, and "unlimited data" (5GB then basically nothing). However T-Mobile makes it increasingly hard to stay with that plan, like eliminating the ability to refresh your plan earlier than 30 days, and for the few times I use it as a phone the service is spotty-- full signal, data works fine, but it might take a few minutes for a call to connect, sometimes failing and requiring multiple attempts. Mint might be an improvement.

I was on an even older prepaid T-Mobile plan, and they stopped allowing the purchase of "data passes" entirely. I was also seeing call reliability problems in urban Southern California.

If you have a spare phone, I highly recommend a Mint trial. There is a 7-day starter pack, which gives you two SIMs and lets you test briefly with a temporary number and then reactivate on the second SIM if you wish to port in your existing number. But, I thought the $45 pack was so cheap that I used it for a longer 3-month trial, after which I was happy to keep it alive as my new main service.

You also have to think about whether you care to keep your old grandfathered plan, i.e. on a backup phone. I kept mine because I can keep it alive for $5/year if not using minutes. But, I imagine I may drop it some day if it cannot be relied on to actually place an emergency call...