Simply Superb. There may be no colour and it can be slow at times (in comparison to other games at least) but has a lot of heart and emotion. And frankly, that's all the game needs.
The design of the game is simple but engaging.
The controls are ok, although I found myself press space to jump a lot at the beginning but that's more me the a fault of the game because I stopped doing it after while.
One critic is that I found myself saying "this to going to be like the film "Iron Giant"" and I was sort of right. Although thinking that, the end still had a lot of emotional punch.
My opinion on the slow gameplay:
The movement of the Guardian is fine as it is. It's slow, hulking movements gives it a sense of scale. It looks and feels like a giant. And I didn't see anything wrong with the movement of the boy at all. He moved as I expected he would and almost the same as other games.