I haven't tried everything but the main mode. I will do them maybe later today. I find that in most puzzley games, only the main mode is worth playing, an the others are just half-assed, usually annoying rule tweaks.
My general assessment of the main mode is in line with Corny's, and a bit with demi's. The game is so much of the same thing over and over again, and once you adject to each level's quirks, it's so simple that I could believe one could play endlessly. It's not like a normal puzzle game with simple rules, since in those, living with the consequences of your actions ultimately is what makes the game get more difficult. Aside from picking up speed things, there is no consequence on the difficulty in this.
I dunno, maybe one of the extra modes does get it right. In the main mode, it more feels like part of a game than a full game. It's more like a bonus stage for a game than a game in and of itself.