We got this bouncer when he was four months old and it was perfect timing. He was absolutely amazed at suddenly being able to control so much about his position. He can turn around, bounce up and down at whatever pace he wants, stop his motion completely, and even scooch over to touch the painting on the door. He would often start squealing and experimenting with all kinds of noises. It seems the new physical possibilities stimulate him mentally as well.
Here is the website for Merry Muscles