Though I’m not involved in a capella myself, “a capella rush” sounds like a pretty apt way to describe the process my friends went through.
I’ve never heard of Sh’ma Capella at all, and the Madrigals–which I’ve only heard mentioned a couple times–are more of a student-directed choir than an a capella group per se.
All the a capella groups have a significant social component, and that strikes me as a plus, not a minus.
Besides Amherst Dance, you should check out DASAC (“Dance and Stepping at Amherst College”). Really active group that puts on a couple very well-attended shows throughout the semester.