I've been a google FI customer for about a year. I use wifi at home and at work, so I don't need a lot of data. I signed up for 1 gigabyte per month and use less than that most of the time, so around $30 per month for me including taxes and fees.
Connection is good most of the time. I live in a big city, so this may play a role. Still, for example, without wifi at work, I'd be struggling to connect to the cellular network on a high floor.
International roaming is awesome. If I travel for a few days, I don't bother buying a local sim card. If it's a longer trip, I'll buy a sim card to have a local number, but still, it's nice to be able to connect at the airport and get an uber/use a map even before you get a local card if you need one.
My biggest problem with Project Fi is limited selection of phones: Nexus 6, new nexus 5X and 6P and Pixel, that's it. Not bad phones, but there are better Android phones on the market when it comes to value for money. If I break my Nexus 6, I don't know what I'm going to do. I'm too frugal to buy an overpriced Pixel. 5X and 6P are already kind of out of date, not the best value for money, not good battery life as far as I know. If Google doesn't start offering affordable new phones, I'm going to switch at some point.