We have used one, dd wasn't a big fan but ds who is the same age as J loves it. We make sure that the setting is the right height and he bounces flat footed for up to 15 minutes every couple of days. You can get some bargain ones off gumtree or the trading post
Same here. DD1 loved it and she bounced flat footed. Spent about 10 minutes a day in it.

I have one you can borrow if you want to try Jett in one without spending money.