We registered for the Einstein jumper when I was pregnant and actually ended up taking it back and getting the Fisher Price Jumperoo after talking to my friend (thanks Kirsten!) and reading some reviews. The last few weeks we've felt Jonah has had good enough head control to sit in it. Its hilarious..hes can't even touch the ground, so we' ve put our wedding album under him so his feet will touch.
The more hes been in it the more he seems to enjoy it. I really like this one, because it doesn't take much to get the jumping going.