Ron Amadeo, writing for Ars Technica. I’ve bolded key aspects.
It’s hard to label any one service as “cheaper” than another—it really comes down to what fits your usage patterns. Project Fi’s pay-only-for-the-data-you-use policy has the potential for savings, but if you use roughly the same amount of data per month and can perfectly fit yourself into a plan from another prepaid carrier, you have a good chance of saving money elsewhere.
Project Fi is great for people with fluctuating data usage though. Take me for instance: most days, at home and at work, I’m on Wi-Fi, with barely any data usage, but there are those months where I travel a lot, and then my data usage spikes. Project Fi would give me money back for the low-data months, while flexing to a larger plan when during busy months. For a person like me, it’s perfect. I don’t need data all the time, but when I do need it, I need it to be fast and plentiful.
Still, price isn’t the only thing to consider when choosing a network. Fi also has a lot of great features that don’t show up in the text/talk/data breakdown. Project Fi’s combo Sprint and T-Mobile plan should have better coverage than either Sprint or T-Mobile individually, and things 3G international data will be a big deal for some people. You’re also getting the ability to send calls and texts to all your devices—it basically has a (hopefully) non-terrible version of Google Voice built in.