Ds loves his - calls it his "flying" bike.
Got it for his 2nd birthday (or might have been the Christams just after) and wasn't interested in it for ages. We would get it out every so often and then last summer, when he was three and a haff, it just clicked. he is now really fast on it and goes zooming down hills, free wheeling (bit of a worry, as it doens't have brakes!).
He is so fast now, that when he uses it to go to nursery, we have to go on our own bike alongside him (him on pavement, one of us on the road). But it is also good for teaching him orad awareness - stopping at each of the junctions, checking whether it is safe to cross etc.
He got a bike for Christmas and although he will go on it, he finds the "flying bike" much easier, so he still insists on using it to go to nursery.
There is a queue of people at nursery waiting for him to complete his transition on to the "real" bike, so that they can borrow the flying bike.