So .. I'm a professional scientist. ACM isn't a fly-by-night operation. It is very similar to IEEE in terms of stature. In CS, I consider ACM conferences slightly higher tier but this is not a hard and fast rule.

Yes .. they do send a lot of material I don't care for. But if you are a student, it is definitely cheap and worthwhile to join. As a professional, you have to make your own call .. I pay for it because I feel the money goes to conferences, student subsidies, etc. (not really charity but I feel like I am paying it forward). You get discounts at conferences but my employer pays for those anyways.

For what it's worth, I didn't have any issues when canceling my membership. I did get some emails for a while, but those tapered off or were easy enough to unsubscribe from (link in the emails). Regarding auto-charging the CC, just don't let them. There's an option to store your CC credentials and automatically renew (I believe the default was to not store and to not auto-renew), you can either not store them, or not automatically renew, or both. I received a notice in September of every year to renew my membership, with a deadline of October or November. That was a bit nagging, but not terrible.

I did find it difficult to unregister from ACM SIGs (Special Interest Groups), had to press on through several screens to get to one that let me unselect them. That was obnoxious, but only resulted in my being a member SIGGRAPH one year longer than I desired and being out $20 or so, interesting reads but wasn't an active pursuit or career-wise applicable topic anymore.