Having just purchased a Schrade Tactical Pen (mostly for the heck of it)
Having been trained in the Mini Baton, (Kubaton) I see this as being very similar tool for self defense, but the primary reason I carry a pen is to write things down, and find the tactical pen not well suited for that application, it does indeed write, but you have to unscrew it, make a note or 2 and screw it back together, which is a bit of pain, as a regular pen is just plain easier to use for that intended purpose.
Now really the tactical pens are a disguised version of a weapon (ie: a pen)
Now I have a friend who has worked in forensics, and pathology, and other research, found that regular old Bic pens have the deepest pentration in a human chest wall of about any pen on the market.
So really about any pen does a good job of writing, and about any pen can be pressed into service as a weapon, and for the most, regular pens are more affordable, and do the primary job of writing a lot easier
L-R Bic, Free Glock Pen, Schrade Tactical, Zebra F-402
Cost-wise the Bic runs about 12-15 cents each, the Schrade was $24.95, and refills run about $1.50 each