I signed up when gears 5 came out, intending to play that and blair witch. thought gears 5 was pretty mediocre, glad i didn't buy it out right. currently game pass is probably something i sign up for 1 month at a time to play through a game or two. dont see it as something i stay subscribed to full time. maybe in the future if games i want to play are released every month. but xbox doesn't have quality releases coming regularly enough yet, i hope that changes next gen.
They do add new games every month, and rarely have I seen them release something I didn't care about; there was at least always 1 game worth playing, even if it was an indie game.
I actually find myself sometimes lowering game difficulty to finish faster as so many new games get added and sometimes removed that I wanna make sure I don't miss the chance to finish it. Also, with work and kids a big AAA game can easily last me a month, so with indies and whatnot + games that have multiplayer (so lots of replayability) I don't see myself ever having down time and needing to cancel. I've pretty much given up on buying new games at this point; so much content from MS that I have enough to play, and if a game will not show up on the service by the time I can play it it'll be on sale anyway.
Obviously if you are single and without kids and can play 40+ hours a week maybe you won't find enough content for yourself; however I think with busy lifestyles that people have these days and the aging community of gamers, this type of service is a godsend as the 16.99$ it'll cost me in more than 2 years is easy to fit on a monthly budget, gives me Gold and games on PC + Xbox, and enough new content to keep me busy and make me save money in the long run by buying less games.