My son got this for his first birthday. Now, just over 2, he finally puts it together on his own, making working gear set-ups. But he loved it right away, and quickly learned to turn the handle if we set it up. He also played with the animals and pieces just as interesting objects at first. Then he worked hard to master the hand-eye coordination to fit the animals and trees into the little holes on the blocks and gears. He also uses the animals with playdough (sticks them in).
Considering the heavy use this toy has had by a young toddler (who is pretty rough on his stuff), I am impressed with how it has held up. Recently my son snapped off one of the posts that attach the animals to the blocks or gears. But he bends them and otherwise abuses them all the time, and there are 7 more animals.
Adults love this toy too, they always want to set up their own gears with it. Its colorful and fun, and its like a puzzle and a building toy combined in one. I am thinking about buying some of the other Georello sets (gears, marble runs) because I think they can all be used together.