How addicting? SO addicting.
Also, I broke the scoreboard. I've written up a medal guide for the game, and I included a gameplay video that shows how to get a really high score, as well as an aftermath video showing what happens when you get a 32 creature combo. It's at http://dimono.newgrounds.com/news/pos t/526392 (sans spaces, obviously).
As for constructive criticism... well, I have none, because aside from the restriction on the NG scoreboard this game is exactly as good as it can be. The only thing I'd maybe request is a quit button in-game, so if you see your game is going nowhere you don't have to wait for the screen to fill up.