I must say this game crept up on me, but I suspect that only served to improve my experience of it as it really was such an unexpected delight.
First off, the game truely is beautiful. Yes it's 2D, with no options for improvement or scaling with hardware but this just doesn't matter. Everything in the game is fantastically realised, the colours are vibrant and vivid, the lines are clean and the style of the game is incredibly distinvtive. Each world has its own theme and this is just the icing on a very beautiful cake.
Soundwise the game is fairly straightforward. A simple musical score bobbles along nicely as you play. It never really stood out at any point but provided a pleasant backdrop to the game. The 'Goos' themselves make a range of endearing squeaks and yelps. The sound is all there and of a pretty high standard but because the game is so absorbing it doesn't really stand out and just sucked in to the subconscious.
Those 2 aspects of the game clearly have high production values but the gameplay is really what makes World of Goo stand out. The game shares a lot of its basic mechanics with Lemmings: X blobs start in one area and using their abilities you have to build your way to the exit pipe and at least Y blobs have to survive to the exit. There's a good sprinkle of Portal in there too, with the wry humour from the Sign Painter and the physics based puzzles. And while it takes the best bits from those games it then runs with them.
You start a level with a small frame and a certain number of goos. The goos have a cunning trick to aid them in their quest to cross all these hazard filled areas. If you pick them up and hold them near a frame, formed of other goos, they will form links with the others. Most form a maximum of 2, but there are different goos that form anything from 1 to 4 or 5 links. It's this cunning property that enables the humble goos to reach their destinations. You build bridges, towers, arches; whatever you can to form a pathway to the exit pipe. Obviously they're not going to make it easy for you and you will encounter all sorts of problems along the way and some of them are truely devilishly tricky. If you're really hardcore each level also has an OCD (obsesive completion distinction) challenge to complete the level in a certain number of moves, or rescue at least so many blobs. They are exceptionally tricky and often require significant lateral thinking and a lot of replays to streamline your method (much like the gold challenges in Portal!).
The game is incredibly simple like all good puzzle games, but there is a definite challenge there that makes it a compelling and engaging game to play. The simple presentation of the game, gentle pace and beautiful artistic direction make for a wonderfully refreshing change from all the unnecessary and laboured attempts to generate 'depth'. Also, the game is, in my experience, completely bug free. Which for a game programmed by something in the region of 2 guys with an artist is a hall of an achievement,
The game is pure, simple, brilliant. Well worth the money, especially to support the talented creators of this beautifully unique game.