And when I first got online in the early 90's, my ISP had an additional subscription option to get real newspaper-type articles delivered in a special usenet newsgroup hierarchy (can't remember the name of the news service itself though).

What do you mean "used to"? Usenet still exists.

And how good is it?

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?