For two lines, you'd pay $35/month ($20 for the first line and $15 for the second) plus $10/gig (so 5 gig would be $50 - I think Esmjb made a typo with $5). The maximum is 6 GB; after that, there's no extra charge so you have an on-demand unlimited data plan for $60 + $35 for the two lines. Generally, data has been at 4G speeds in cities and 3G elsewhere (and lower in the boonies); there may be some countries where that is not the case, but as far as I can tell, the speed is pretty good in most countries - so it's far better than T-Mobile's 2G limit. Texts and calls are free within the US but generally $.20/minute outside the US (with I think some exceptions). Calls to the US from abroad are free as long as the call is on wifi.

In Brazil I made the mistake of trusting that whenever I was on a fast wifi connection, the call would be routed via wifi. That is not the case, and Google charged me a lot of money for calls home that could have been handled over wifi but weren't. They told me I have to turn off the cellular connection (basically, go into airplane mode and then turn on just wifi) to force the call onto wifi - instructions that appear, as far as I can tell, nowhere on their website. When I started doing that, the call quality was actually better than over the cell network.

Aside from this one annoying quirk, I'm pretty happy with Fi. I've used it in three countries so far, plus the US, and it has been really hassle free. Of course, the service works best with the limited number of phones they support. Those range from budget models to the latest Pixel and some fancy LG phones. Iphone users are pretty much out of luck. (Although it's technically possible to use iphones on Fi, there are a number of limitations involved.)

(By the way, for every line, Fi will give you an additional data-only SIM card to use in whatever device you like, at no monthly charge, aside from the data cost of course.)

Edited to add: There's a huge thread about Fi and how well it works in different countries over in the Travel Tech form: Google Fi: Anyone care to post their experience? .