I really wanted to use Project Fi. I'll just admit it: I wanted it to work because it's Google and it's different! And it is both those things, but that was not quite enough reason to stay with it.
I signed up for Fi about a month ago, and bought a nicely cheap Nexus 5x when they were selling them for $200 with sign up. I think that promotion is over now; however, I got to keep the phone when I cancelled, and I do like the phone a lot.
The real issue is that I'm not doing a lot of international travel right now, and I have the world's cheapest (as far as I know) plan for (non-family) big-data users, T-Mobile's $30/mo "Walmart" plan*, which only provides 100 minutes of talk but 5 GB of data over LTE/4G (after which you are capped at 3G speeds). T-Mobile's LTE coverage in the Bay area is fine for me, and I hardly ever need to talk to anyone when WiFi isn't available. It turns out I use most of that 5 GB every month, which would have given me a monthly bill over twice as high using Fi.
If I was doing a lot of international travel, I might change my mind. I used T-Mobile's Simple plan when I last went to New Zealand ($70/mo) and it more or less worked in most places, but the data rate cap was sooo slow. I would have been better off buying a local SIM (and pointing my Google Voice number at it!). Project Fi uses a 256kbps data rate cap internationally which, although also pretty slow, would have been a big step up!
Generally, I was a little disappointed that Fi seemed to use Sprint a lot more than I expected; given that my T-Mobile coverage is generally good I was expecting to use that more than I did. And Fi's vaunted high-quality WiFi coverage didn't seem to add much to my experience.
Still, it was kind of cool, and I got a cheap 5x out of the experience. And if my wife starts dragging me to international destinations in the future, I'll give some thought to trying it again.
* The "Walmart" plan is available at Walmart or online at T-Mobile, but nowhere else.