As long as they aren't in there for longer than about 30 mins at a time it's ok - but OMG did DS and DD have fun in them! I loved the JJ, it's a crackup to watch their little legs bouncing away.
:yeahthat: They have been around since I was a baby and we never had any problems. As Lulu and others suggested, just use it correctly and they are tons of fun.