hi,im not sure how good or bad,i always thought they were good for them, but my DD loved it from ever such a young age,nothing to do with the jolly jumper but she was late crawling and things so had to go to physio and i asked her aabout it and she said there ok for short periods of time,i couldnt see any harm and my DD just had a ball in it and shes fine walking etc.I got one second hand and it worked fine!