Be aware that $15/mo is promotional pricing. It's more like $23 or so a month after the first 3 months. I use Mint and like it.

I used Mint on and off. The service itself is solid, and the international rates were among the best around. I only left because my neighborhood has poor TMo coverage.

I'll second that Mint is solid. They're a no-nonsense TMO-based MVNO. If you just need text, talk, and minimal to moderate data, and are okay with TMO's hit or miss coverage and iffy building penetration (which seems to have only improved moderately in spite of their band 12 LTE rollout), Mint is fine.

Only two things I noticed when comparing on their website. 1) lock in contracts, and 2) they piggyback off the T-Mobile network while Google Fi dynamically uses the best of 3 networks. Otherwise seems like a pretty frugal option.

>they piggyback off the T-Mobile network while Google Fi dynamically uses the best of 3 networks

This happens only in theory, not practice with Fi. As a Fi launch customer that stuck with Fi through late 2017, Sprint (and later US Cellular) service was always terrible and frequently unusable, no matter what metro area I used it in. My experience improved dramatically when I just kept my Nexus device around as a spare, and moved my primary SIM to an iPhone, where it was permanently locked to using TMO.

Anecdata: Mint has been working quite well for me for about a year, including during a few trips to Europe. I believe pricing gets even a tiny bit better if you buy service for a whole year at once.