My daughter is a D1 gymnast. We have had the trampoline since she started gymnastics years and years ago. It has been a constant source of entertainment and activity for both of my daughters. I always figured that my gymnast is probably the safest person to jump on a trampoline, unlike me when I was young, because of her body and air awareness. It is not and never was a great place to learn new skills, or practice best form for skills, but I have always said that it has improved her gymnastics based on the air-sense alone.
I have never had a net because we bought ours for $25 off of craigslist, and I acknowledge that is a probably a better idea overall, although I've seen so many videos of a net stopping a kid in mid air and making them land strangely, or them falling through/out the bottom, or the net just ripping right open...so there is that.
Anyway, my gymnast is home from college right now, and she's been using it every day for about an hour. It is a really great way to stay in shape, even if you're just jumping. I personally can attest to getting tired after 5 minutes of easy jumping, even when I can easily run a 5k, because jumping and balancing on it uses all of those fine motor muscles that gymnasts have. Oh, and she even invented a jaeger drill this week to help her practice the motion. I am thrilled we have it.