I bought it so I could use my BT keyboard and mouse, and my external monitor with my powerbook when I'm working at my desk (I still use my PB monitor as well).
I'm considering getting it as well for my 17" PowerBook as I want to hook up a 20" Aluminum ACD with a wired keyboard and mouse. Looks pretty cool, so I would get it.
It's basically good for elevating your PB if you want to use it's monitor on a desktop, and also good for keeping it cooler since it raises it up a bit. Other than that it really servers no other purpose other than another cool accessory for apple.