The game is sort of like a bridge building + lemmings type game. Your goal is to get a certain amount of goo balls to the exit point, but you have to use the goo balls to build the structure to get there. Not all the goo acts in the same way (some stretch, some float, etc.) and the structure you build won't be rigid so you often have to get creative about how you get to the end.
So that's the basis of the gameplay, but the presentation is just outsanding (IMO). I really enjoyed the art and music, the balls of goo "feel" right when you're moving them around. The sounds that the goo makes fits, etc. I had an absolute blast figuring out the puzzels because the gameplay is solid, the puzzels are interesting and the setting is pleasing to the senses.
Of course, that's all my opinion and obviously I had a great time with the game I think my only complaints would be that I want more levels and that the resolution is fixed at something like 800x600 or something. The resolution doesn't matter for gameplay, just for native LCD resolution.
There's a demo, so why not grab that and see what the fuss is about?