I originally chose to design out of necessity for a case to protect my Pi. After looking online for a case, I realised it would be cheaper and quicker to build my own. I had seen a couple of lego cases on the internet, so I knew that the size of the bricks would fit with the dimensions of the Pi.
The bricks were already available from my younger years, but finding enough bricks of the same colour wasn't going to be possible so I went for a mix and match approach. After that, it was just a case of trial and error trying to get the sizing right.
The first build featured a set of wheels, purely for novelty, but sadly had to be axed in order for the Pi to fit in a TV cabinet. Several weeks later and the case has reached its final version, after having to adapt the size of the holes in the side of the case to accommodate cables.
Overall I'm very happy with the case, as it was easy to make, keeps the Pi cool all with a playful look about it. I would recommend that people without a case on their Pi consider a lego case, especially if you have some lying around! My Pi is currently being used as a HTPC, running Raspbmc and I have a spare SD card with Raspian loaded on it with web server functionality for experimenting.