I have previously written about how the very small Fisher "Stowaway" model of the famous Space Pen is my constant companion. Three years later, that's still true. There are a couple of misconceptions about this pen, though, which you'll discover if you read through the Amazon reviews for the many (confusingly) product listings you'll find there.
First of all, it's not plastic. It's made from plastic-coated aluminum. But even if it were plastic, which again it's not, so what? It is strong and lightweight and attractive.
It is refillable. I wasn't sure about this when I first started using it, and it's easy to find Internet posts and reviews that say it's not. Apparently, even the Fisher company is confused about this as some people report that customer service has told them it cannot be refilled. They're all wrong. This review on Amazon reveals the secret. Just hold the pen straight up and push down on the tip with some force. The end-cap pops up and after that, Bob's your uncle.
Finally, any Space Pen user will tell you that the first sign that you need a new ink cartridge is that the ink starts to get gloppy. I hate that, and always try to refill right away. I've read that the "bold" and "fine" refills are less prone to this, but I haven't tried them (yet). You'll have to dig a little to find the bold ones, most of the time all you can find are medium.