I wanted a sturdy ballpoint pen to mainly use outdoors and carry in an external pocket, and this, the original astronaut pen and the similar retractable shuttle pens were the obvious options since I prefer Fisher's refills.
There were three things which influenced the choice of this version: It's got a slightly larger diameter, rides lower in the pocket and the most important thing is that the spiral groove near the tip give a much better purchase than the shallower grooving on the shuttle pens.
Both are very well engineered, with attention to things like the spring which you can't loose when you screw the pens apart to change refills and a pushbutton mechanism which always makes the pen ready for writing when you push it (and a small button almost 90 deg from the clip which retracts the writing tip).
I carry it a lot and like to use it even when I've got other ballpoints available. The refills I like to use a Fisher's bold in blue or black, as they write a bit smoother than the medium and fine.
Some basic data (data for the shuttle model in parenthesis): Length: 130 mm (135), balance point 80 mm (85) from the tip, diameter ~60 mm from the tip 9.5 mm (~8), protusion over the clip when carried 7 mm (20). Mass approx 25 g. Force required to operate the pushbutton is ~2.9 N, ~3.5 to retract.
Fisher's web page about Astronaut Pens.