I book flights with Google Flights and other options (Kayak, my credit card’s travel portal, airlines directly), and Google Flights is kind of mid. It’s annoying that Google pushes it to the #1 result for lots of searches that it shouldn’t be.
I guess it depends on what you look for. I haven't bought a flight in a while, but I just looked at kayak and google flights for comparison. Google flights is a much simpler, more direct UI, and it feels a lot more responsive. In particular, kayak violated one of my huge personal pet peeves: page content moving around while it's loading.
That said, the gap is a lot smaller than it used to be. There was a time in the past where google flights was light years ahead of priceline, kayak, etc. in bloat, bugginess, responsiveness, etc. That gap has shrank considerably and it's more a matter of preference today.
