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.