Not Much Fun
I remember playing on one of these when I was a kid as well as seeing my own children try to play with them now. We never bought one (we only played on them at preschool or at a friend's house), and I'm glad we didn't waste the money on it. For a young and light child, it will spin freely, but the child is often not strong enough to spin himself. When he gets big enough to be able to pull himself around, he is too heavy and the sit n spin will not spin freely. When the child is able to spin around, it's something that makes him dizzy and sick feeling rather quickly. It looks fun before you get on it, and it can be fun for the first few seconds. But pretty soon it ends up dizzying and uncomfortable. I've seen several kids get off of them with a rather sick look on their faces and an upset stomach (kids that could get it going faster). It seems like a fun concept when it's made, but it ends up being a lot less fun when you actually get on.