Like any community, that's defined by the people in it. Some groups are excellent, some groups are mostly dead, some display varying levels of toxicity. Overall my experience hasn't been a bad one.

What kinds of groups are there?

Given the lack of popularity and commercial support, combined with the complexity to connect a client with an available server (compared with downloading an app from the store), I'd expect that they are populated mostly by old tech people or passers-by from universities; and any special interest group would have small user base coming from that demographic profile. Are my assumptions correct?