I just bought my second and third Fisher Space pen. I accidentally left my first one in the car I dropped off at Hertz so I bought two more (through Merlin's link). I like to find something that works and stick with it, and right now it's these pens. Sure they write through blood and water, and you can write upside down, but it's really all about the size-doubling form factor. When closed it's about 3/4ths the size of a normal pen, when you take the cap off and place it at the end of the pen the size grows to a normal size. It also doesn't have a clip, which are great if you wear pens in your shirt pocket, but not good when it gets stuck on sheets of papers as you try to pull it out of your bag.
The black metal housing easily beats my previous favorite pen: the Sakura Micron, which was plastic. Though the pen wrote well, the solid heft of the Fisher Space pens