What about Dan Da Dan is giving it such high ratings?

People like it. Personally, I like the story-telling, the characters and the execution.

Do you think I’d find this series worth it?

Hard to tell without knowing what you like. Based on what I’ve seen so far, I guess you’d like it if you enjoy a romcom filled with action and a pinch of emotional moments.

What is the expected age group for the show?

It feels like the target demographic is shounen / boys. I’m older (i.e. seinen), but I still enjoy it. I believe ladies (both shoujo and joshi) would like it too. IMO when it’s good, the demographic matters less; everyone would like it.

the series description gave me a hard pass for the aforementioned reason Felt to me like a bunch of nonsense being yelled at opponents until some cool anime action happened

I haven’t (and don’t intend to) watch JJK for the same reason: it sounds like a typical battle shounen anime that is boring as hell. That’s not what is happening in Dan Da Dan.