A few things to think about...
Are we talking contact-stun, where you must physically drive the stun devide into the perp? Getting that close to someone is not usually a good idea, and I'd much rather be heading the OTHER way...
It is a momentary device. Easily recovered from, especially if it enrages the perp. I've seen me recover instantly from a Taser hit.
Will this be constantly carried in your hand? Will you have a stun device holster? Digging around in a purse, pocket, glove compartment, console, etc. isn't an option when you NEED it.
A lot of people forget to check the batteries/function. It is left in the trunk, glove compartment, nightstand drawer, etc. Batteries dead, corrosion on circuits, etc.
As the man said, the Taser is the only real option out there. Separation of perp/victim, prolonged incapacitation, My last instructor said the civilian model discharges for a minimum of 30 seconds. It is meant to be left on scene while you didi mau. The Taser discharges confetti-like paper with the probes; the confetti is printed with the cartridge's ID number (registered to buyer).
However, just as with any other projectile-firing device, you'll probably be concerned about civil liability for putting a probe in someone's eye, etc. Ridiculous as it is, it's still a consideration.
-josh