I would respectfully disagree with Blades...
The Harrt's rod, unlike recoil spring devices and other mechanical doodads, should not affect the reliability or safety of any pistol, and would not allow it to fire out of battery. The Harrt's rod simply consists of a steel guide rod, identical in dimension to the standard Kahr rod, which is filled with mercury and ball-bearings. It provides inertial dampening of the muzzle flip (albeit minimally) but does not affect the cycling of the slide or any of the timing thereof. It does not "reduce recoil" in the sense that the felt recoil isn't greatly affected, it just keeps the muzzle down a bit.
I have a Harrt's rod in my K40, and have yet to have a malfunction with this pistol. I will say that the effect of the Harrt's rod is minimal, but slightly noticeable to me; it does not replace practice and I would and do practice diligently with mine as it is my primary carry piece, but the small effect it has on muzzle flip allows for slightly faster follow-up shots. If you're hesitating about spending the $75 or so on one, then just hold off and buy more ammo to practice with -- that would probably serve you just as well. Best wishes.
Andrew