I've been using Project Fi for a year and a half now. It's literally cut my two line smart phone bill by 75%.
Coverage is as good as it looks, especially international which was the big draw for me as I was traveling internationally a lot at the time. The WiFi calling fills in the gaps and the automatic VPN for public networks is super sweet.
Migrating to the service was the easiest thing I've ever done involving a cell phone it's all web based and there isn't any BS.
The way the process works is you fill in your billing info and number you want to migrate. Pay for the phone and request the SIM. Google sends out the phone and sim. All you do then is get some WiFi, activate the phone, and sim then cancel your old plan. Your old number becomes active on the new phone about an hour after you activate the ProjectFi sim.
Here's my referral code: https://g.co/fi/r/YC031K It's good for an unlimited number of sign ups so I don't care who uses it. It's a 20 dollar credit after 30 days for both parties (just getting that out there.)
The phones: They're overall they're very good. Android is a great system when it's actually updated and not full of bloatware. The Nexus phones have a cult following for a reason and they've lived up to the hype for me.
That said: The Nexus 5X has an issue where it will randomly bootloop. Project Fi replaced my wifes for us free of charge so I didn't have to deal with LG over the issue. I'd still stay away from it as it's showing it's age and coming to end of life after the Oreo update minus security patches. Even with that problem it's been pretty good to me so I can't complain really at all. Great camera and generally good experience.
The 6P supposedly has a problem with the battery but I don't own one so I couldn't tell you how wide spread the issue is.
I don't know much about the new Pixel2 phones yet as they're due to be announced in a few days but the Pixel is generally pretty well regarded.
Other bonus items:
Everything (phone calls, SMS, messages, voicemail) ties pretty neatly together through Google Hangouts as well which is really convenient.
Billing is ultra transparent which was a nice change from Verizon. If you want to see what one looks like I can pull one of mine. The ProjectFi app has up to the minute official data usage so you're unlikely to ever be surprised by the bill if you check it from time to time and also provides a mechanism for technical support which requests that they call you.
Come to think of it I don't have any complaints at all really which is saying something since I despise every other phone company I've ever dealt with.
Oh and there's no charge for tethering which is useful for info you don't want transferred over a hotel or airport network. Verizon and AT&T can suck my butt hole over the fights I've had with them on that front.