I had one, it was awesome. It was round, and we lost the spring cover VERY quickly. A kid got his leg stuck between the mat and rail, NBD, walked it off. I flew off it once, was OK. Jumping off things onto it = bad idea, you go up almost as much as you go down.
For some reason, water increases the bouncy-ness SUBSTANTIALLY. Also, water hoses are just fun in the summer.
As a kid, I would have preferred a gymnastics trampoline (those yellow rectangular ones with red lines) they are INCREDIBLY bouncy compared to the walmart or what have you brands. They are seriously dangerous if a kid is into pushing his/her limits, I seen a kid throw a triple backflip on one of those and almost crumple his spine.
I learned a lot about being in mid-air from it. Also how I learned to kickflip, heelflip + body varial, double kickflips, and tre flip. Just take the trucks off the skateboard, or the tape on edge of snowboard/skis.
They can also build up a SERIOUS static charge. Friends and I would rub against the mat, touch hands (there would be a small shock as we did this), then the person on the end would touch the metal frame, and at night there would be a huge visible shock, and everyone connected would feel the shock. pretty neat.
it was a lot of fun.