This is quite possibly the best user created level thus far (at least from a technical/artistic view point). If you have a copy of the game, go and play it now, don't spoil the experience by watching the video.
If you don't own the game, go buy it. But, failing that, here's a video of the level: http://uk.youtube.com/watch?v=FEEJbGJqCh8&fmt=22