A kubotan can be hugely effective, but punching with it in the manner of a roll of quarters is not recommended because you're likely to hurt your hand.
Kubotans are effective because of physics—puts all the force into a small surface area capable of delivering a definitive strike.
The best method to use a kubotan practically is to use a backhanded grip, a form of hammer fist.
Here, you're not generally hammering down, but in a horizontal manner. The main targets are the kidneys, the throat, the solar plexus, and the temples, but any strike anywhere on the body in the manner will be painful, and certainly give you a moment of advantage for followup strikes.
- The caveat here is that it's a close-range weapon, so if you don't have the defensive skills and footwork to get inside and stay balanced, you're unlikely to be able to wield it effectively.
(Another advantage to kubotan is it carries less potential legal liability than a knife if you seriously injure or kill your opponent, and, typically, you don't want the blood of the type of person who attacks people in the street getting on you, especially if you get cut.)
If you have sufficient training in any martial art from a good instructor, and are worried about defending against a bigger, stronger, or more skilled opponent, kubotan is a greatgreat street weapon b/c it fits in your pocket.
- There are also versions that include two blunt finger spikes, and those do facilitate punching b/c all the force is concentrated in the spikes.
Doesn't protect your fist in the manner of brass knuckles, but concentrates force in a smaller area, reducing impact to the knuckles. These types of kubotans also allows the hammer fist in addition to the punch, without changing grip.
A skilled practitioner can also use it for a thrusting strike from the top of the fist, such as under the chin where the glands are, or into the ears or temple, but it's more difficult and so not recommended as the go-to.
Never underestimate the utility of blunt-force trauma.