Love them! My kids love them and literally will spend hours on them. I do get on too sometimes. Trampolining is great exercise as it exercises ever cell in your body, even your fingernails! You don't have to go anywhere to do it, and it feels great!
I have a trampoline. It's down near the washing line, so I put the kids on it while I'm hanging out washing so I know where they are and that they're out of trouble. They love it. I try to get them on it for a while in the morning to use some excess energy so they can concentrate to sit down and listen to a story afterwards. It's great exercise and they sleep better.
I loved my trampoline as a kid, but I'm glad the new ones are a bit safer than the one we used to have (no net and dodgy padding so you could fall on the edge or the springs or get bounced right off).
I used to have a small indoor trampoline when I was very young. Then as I got older I went to a sports club during the holiday and used a proper professional trampoline. It is fun for a short time, but when you start to get tired it loses it's appeal.
I love trampolines but had a bad accident on one when I was a kid so don't go on them so much now. My little boy loves them but I do get anxious when there's too many kids on at once, it always seems to end in tears.