I've decided to cancel my subscription on this.
It has been phenomenal value for the past two or three months, and I've tried out a ton of games I've been curious about for a while for very little outlay.
There's not a lot on there I'm massively interested in now, though. Some things I wouldn't mind trying out, maybe, but the real killer for me is the Windows Store gamefile system.
I wanted to uninstall a couple of games I've been playing to make space for new ones, but also wanted to back up my saves with the idea I might go back to them at some point. But for both there is no way to even identify where the saves are, as they're somewhere in the obfuscated file structure MS uses for its Game Pass and Store games. So I have to either finish them now and be done with them, or just kiss my progress goodbye. This is too annoying, and it has put me off trying any games I might want to play over the longer term.
There's really no excuse for locking users out of the gamefiles like this. Not only does it make modding difficult, or even something like editing ini files often impossible, but I hate not being able to have control of my own saves.
Also, the wonky download speeds
@Cully mentioned above are indeed a bit pants. Every other game client gives me a solid 37MB/s. With Game Pass it's a constant second-by-second fluctuation... 5MB/s...3... 7... 15... 8... 25 (woohoo!)... 3 (dammit!)...
I might be tempted back if they get some super new games join the Pass, but for now this is a deal-breaker for my current sub. Hopefully they fix a few things like this during the course of the beta.