Putting it bluntly - I could live in this place!
The streets are a maze of sensory pleasures, constantly stimulating you as you move through the space.
Lonely Planet have a great little write up on this place with directions and comments from other travelers.
How do you get here - get a cab! its so cheap and it saves you a lot of hassel - Its about a 20min ride out of the heart of the city and will cost you about 70RMB (around $10AUD). Well worth a visit and save a whole day for it! It normal opens around 10am and its completely free to get in. There are some exhibitions though, inside the complex, that charge a very small fee of between 6-12RMB (about $1-$2AUD).
If there is one thing you must not miss in Beijing... the 798 Art District is it!