I really liked this. It was decently challenging, and felt vaguely like a 2D Banjo Kazooie sort of game. The secrets were hidden well, but fairly enough for me to get all of them.
My only real complaints are that there should have been at least one checkpoint in some of the longer levels, and that there wasn't enough variety in level themes. I'd love to see a sequel with more varied worlds, and more stars per world.