Project Fi was a great deal when it was first introduced, minus the fact that the only compatible phone was the Nexus 6. Now that carriers are re-introducing 'unlimited' plans, Project Fi's $10/GB pricing tier isn't quite as appealing as it once was. Fi has now introduced its own version of an unlimited plan, called 'Bill Protection.'
Project Fi has always had the same pricing scheme - you pay $20/month for unlimited talk and text, and then every GB of data you use is $10. You're only charged for what you use, so if I hit 2.5GB in one month, my bill would be $45. This works out great for users that are almost always near a Wi-Fi network, but not so much if you use plenty of cellular data.
Project Fi will now cap your total monthly bill at a certain amount, based on how many people are on your plan. If you have one person, your bill maxes out at $80/month. Then it's $135 for two people, $170 for three, $205 for four, $240 for five, and $275 for six.
Single users are now charged a maximum of $80/month, no matter how much data they use.
Put simply, single users are now charged a maximum of $80/month (plus applicable taxes and fees), no matter how much data they use. It's very much like unlimited plans from other carriers, but you still get unlimited tethering and full-quality media streaming. And if you still only use small amounts of data, your bill stays exactly the same. Project Fi also says speeds are reduced once you pass 15GB; that's a bit lower than the threshold for other unlimited plans (AT&T's and Verizon is 22GB, and Sprint's is 23GB).
With Bill Protection, Project Fi is once again competitive with major carriers, but there are still a few downsides. The lack of available devices is still a major problem; the only real choices are the Pixels and the Android One Moto X4, considering the Nexus 6 is old and the 5X/6P suffer from major hardware problems. Hopefully Google will continue to expand Fi's mid-range offerings, and maybe offer some lower-end devices, like the Moto E4 Plus.