I have got one for exersize, but have taken to sitting on it while watching tv as all a bit uncomfy in the girlie bits at mo, and it's not got loads of air in it, so soooo comfy, we are a seat short anyway , with a 3 seater couch and a 1 seater , and 3 kids, oh and me, so it's working well for everyone else too!
I did find my legs ached and back to start , just sitting! I think it works your core stability muscles just sitting on it, so build up using it for short times only eg 10 mins a go till your more used to it.