I really love this game. In fact, this game is the reason I joined Newgrounds. There's one issue, though. I had already completed all the game's levels, all but 4 of which I had beat within the Star Moves. When I registered, my achievements were not kept, but the levels were. Somehow, this resulted in my being able to redo the Completion achievements, but not the Star completion achievements.
I.E. When I completed the first third of the levels within the star moves for the second time, I only got the "Black and White" achievement, and not the "Twinkle" achievement.
As to why this game is so great, you really just have to play it yourself. I haven't played a vast collection of games, so I don't know if the idea of a negative space puzzle game is actually new, but it's certainly interesting. I remember playing this a few years ago and, while some of the puzzles were a lot more rage-inducing to me at the time, the game was still completely fun and addictive. I recommend this game to anyone, at any age, especially if you enjoy puzzle games. The theme of escorting tiny, adorable-looking monsters to their heart-wrenching destinies is a nice touch, too.
However, another small issue I had was with the few levels that require more hand-eye coordination and/or speed with your trackpad/mouse than they do puzzle-solving skill. In the end, they aren't all that hard (although I think I had a lot more trouble with them when I played this a few years ago), but I feel they detract from the overall experience.