My mares sire is out of a Jazz mare. Switched on, but workmanlike is how I’d describe. I am noticing mine getting a little more spicy now she’s had a couple of months off to grow a bit. I wouldn’t buy a horse bred like this if you’re banking on it becoming less sharp.
I personally enjoy Jazz offspring / relations, but they’re not for everyone .
I have a Westenwind x Jazz mare, she's 16 this year, bought her as a 6 year old. She used to be really spicy and sharp, but she's gotten milder through the years. Went out hacking this weekend in a group, and she was the best behaved horse! She started maturing around 9 years old, but she was a very late starter. She wasn't broken in until 6 years old.