When I last used Fi, before it started making a periodic clicking sound on all incoming calls and disconnecting within 1 minute, I never once saw myself connected over Sprint - it was always T-Mobile. Unless things have seriously changed over the past three years, Sprint just has virtually no coverage.
Google never could help me with the aforementioned clicking-and-then-disconnect problem at the time, which both my mother and I experienced, so we had to drop them. Shame that, too. My monthly bill was usually less than $30, and hers was rarely over $25. I do wonder if they ever improved the call latency issues though.
Remember to get the correct model...
For the Samsung phones, make sure that you either get the universal US model ('U' suffix) or the Canadian model ('W' suffix) since the US carrier branded models typically lock out other carriers' LTE bands and often lack support for rival 2G/3G protocols.
Project|Google Fi is cheap, and has hassle-free international roaming and hassle-free support. Here in the US, Fi is a steal: $20/month for unlimited talk/text for the first line, $15/month for each additional line, $0 for a data-only SIM, and $10/GB/month (with a refund for unused megabytes!). The service works in almost every country in the world. And the Fi app has a dedicated button that dials tech support, where friendly agents speak unaccented colloquial English and solve your problem without trying to upsell you.
I've used Fi for several years, but I've never recommended it to my friends because the phones have been so flippin' expensive. This is good news.
Great if you don't need data.
If you use 6GB or more a month, this plan is actually as or even more expensive than plans from T-mobile and the like.
Sigh, what is the point of all these fancy new networks they keep rolling out if they try to penalize you for using them?
I'm sticking with my un-throttled T-Mobile plan. I used 30GB+ last month. I don't even consider WiFi unless my cell signal is bad.