On the Fascination, it was very calm. I think Serenity just means no kids...not obnoxious adults! Although, I've heard of people bringing kids there as well.
My biggest complaint is that the Serenity Deck suffers from the same problem as the Lido deck; chair saving. I am a morning person, I went for a walk early one morning around 6am. I walked thru the Serenity Deck to find only one other person there (reading a book). However, every single deck chair and clam shell was "saved" already ( I bet from the night before).
On the Glory, the Serenity Deck in on the same deck as the kids' Camp. The kids can stare at all the adults' doings directly thru their play area windows all day long.
And yes, the most obnoxious people I find a cruise, are adults not kids.