I've used the service twice in the past; both times was due to them offering me a free trial. I found the service to be functional, but I did experience a bit of lag and input delay. If the lag gets too bad it kicks you out of the game. I should say that the second time I used the service was much better than the first time, and I'm sure that by now it's a fairly smooth experience, and since then I've upgraded to a 250 Mbps connection.
Usually if there's a game I want to play, I'll just buy it. If I can recall correctly, I opted into the free trial prior to the release of MK11 as I wanted to play MK9, or maybe it was MKX. During that time I also tried a few other games, but overall didn't spend very much time with it. As a kid, I would have loved having a service like PS Now. Getting to play new games was few and far between, and the prospect of having a library of games to play on demand would have blown my mind.
For anyone who hasn't played God of War yet, this is an insanely good deal if just to experience the game. God of War seems to be the poster-child of PS Now, as pretty much every GoW ever made is available on the service.