I like derringers. My first backup gun was a High Standard in 22Mag. Back in the 70's you could hardly find a cop without one. I read a story about a guy who thwarted a kidnaping by killing his two attackers with one! So I guess it does what it was designed to do. I still have mine and shoot it a coupla times a year. I wouldn't be afraid to carry it today. While it might look like it's the same size as a Kahr Mk40, it's actually much thinner and lighter, easy to conceal and a delight to carry.
I had a Bond Arms in 40S&W. It has a rebounding hammer and a crossbolt safety and is very accurate, after you figure out the trick to shooting one. (Pull the trigger DOWN and back, not just straight back) It was almost the same size and weight as a Kahr Mk40. The Bond is a well built, quality gun. I never got a set of 45/410 barrels for it, but one set up so would be a dandy fishin' gun. (I traded it for a Remington 7600 '06 Carbine. If it had been the 45/410 I probably wouldn't have.)
I lucked on to an American Derringer DA38 a while back and it's a nice little gun. Built along the lines of the High Standard, but has a much nicer trigger pull. It even has a thumb safety that easily flicks off and on in the manner of the 1911. A little thicker than the High Standard but with it's aluminum frame it's light and conceals easily.
The American Derringer M1 in 45/410 is just a hoot to play with. Yeah, you have to remember to put the safety on before you load it, since it doesn't have a rebounding hammer, and yeah, the 45 Colt bullets keyhole. But at 10 feet who cares? It never was a target pistol. And shooting shotshells out of it...well, it's minute-of-snake, not a skeet gun.
Derringers have their place if you're willing to train with 'em.