The Diamond 580 is the classic TWSBI pen - the culmination of the line they've been gradually improving for years, and it's really good now.
The piston filling mechanism works well and smoothly, and holds plenty of ink. The clear barrel means you can see when you're running low well in advance, so you don't run out of ink unexpectedly. The style is perfectly TWSBI - crystal clear, cut into a facetted pattern that catches the light, giving the pens their name. The steel nibs are reliably good, and the size and shape is nicely comfortable for most people. For a smaller pen, see the Diamond Mini.