I joined the ACM a month ago or so because I was submitting a poster to an ACM-run student research competition. There was a requirement that, if the poster is accepted, you have to be an ACM member. But I decided I would join anyway. The student membership is relatively inexpensive, I thought, and I may as well try it out.
When I received my first Communications of the ACM (CACM) magazine, I was blown away. It was easily three times thicker than the meager IEEE Spectrum magazine. Whereas both live in my bathroom, I find the CACM has hours more reading material in it than the Spectrum. And it is interesting, and enjoyable.
Take, for example, student and faculty attitudes and beliefs about Computer Science. It is about how students' beliefs about computer science as a discipline and as a way of thinking converges with or diverges from faculty consensus. Where the students diverge from faculty, there is a (arguably) a curriculum gap that needs to be fixed.
Also, the ACM membership card is nicer, and the website is cleaner, faster, and much less buggy than the IEEE website. It is clear that the IEEE website was written by electronics engineers, not programmers!