I have two. One Johnny Jump Up and one Fisher Price. My son LOVES them. He will sit for a long time (long for an 8 mth old anyway) and just bounce and bounce. (Did I mention he loves them?)

My brother was a doubter, but when he was over with his daughter she wanted to get in so he let her. He went the next day and bought on.

I would add, though, not to leave the child alone. Some how my son managed to slide through one of the leg holes the other day. He was just bouncing around in a little circle. (His big brother was laughing too hard to help).