A surprise hit and here's why:
The game doesn't have a big epic story. You're really just going around a planet controlled by this Goo Corporation and each stage has its own separate objective like fixing some massive statue-shaped power plant or launching the new "Z product". The storyline is filled with humorous details, however.
The gameplay is what makes this title so fun, despite its simplicity. In each level, you have a certain number of goo balls at your disposal and you can connect them to each other to build structures. Alright, at first this doesn't sound like much fun since all you can think of building are towers and bridges. However, you will find that each level manages to challenge you differently and the solution is always unique. Plus, different types of goo balls also make the game more varied, requiring the user to consider which is best suited for what purpose. A puzzle game, basically, except it's not repetitive.
The graphics are okay. The installation is short and the game fits on a CD-ROM but you can still expect some very nice art throughout the game (and humorous, as always). Good music too but it gets re-used sometimes.
My only issue with the game is how short it is. It's not THAT easy but there aren't enough levels. Perhaps that's the price one pays so that each level is unique. However there is some replay value with the Tower of Goo "mini-game" where you can use the goo balls you collected to build a tower as high as you can. The fun feature is you can compare your tower's height with others over Internet. And the goo count is limited at 300 so people who started playing months ago have no advantages over those who just started.
In short, World of Goo is a fun game for anyone who enjoys puzzle games even slightly. Don't expect it to last more than a week-end though.