As with most other handgun/caliber choice issues, this one is pretty much influenced by subjective considerations.
I don't own Kahrs because I dislike their triggers, but I occasionally get the opportunity to shoot them since a small number of our folks own them. They seemingly aren't as popular as other makes/models as off-duty weapons in my area, nor do they show up very often in the CCW classes.
I know a guy in my unit who has owned & carried a K40 for a number of years, however. He said it's always been reliable for him, and he's used it for one of his IDPA guns for a while. Due to the overall weight, though, he's replaced it as his primary off-duty weapon with a S&W 442 .38 Spl J-frame. FWIW, while I've never particularly cared for the Kahr triggers, I've found his K40 to be a decent pistol. Reasonably accurate, too. Just heavy for its size, although the increased weight seems to make the K40 a controllable pistol.
Another fellow brought a MK40 through the range one day. He'd just bought it and hadn't yet had the time to shoot it, so I gave him the opportunity to run 3+ boxes through it before he tried to qualify with it. It had more recoil than he was expecting (carrying a full-size semiauto for a duty weapon, he might've had less experience with compact pistols), and he invited me to help him break it in.
I found the little pistol pretty heavy for its size, and quite frankly I prefer my G27 from the perspectives of controllability, accuracy, ergonomics and overall weight for such a small pistol. As much as these things rely on subjective considerations, however, that shouldn't be given more weight than anyone else's opinion.
The MK40 exhibited normal functioning during the 'break in', and the new owner qualified with it. The next time he came through the qualification range, though, he still wasn't as pleased with the controllability of the little gun as he'd expected. I somehow doubt he'd been doing a lot of practice, though. From my perspective, his primary use of the longer magazine seemed to negate the advantage of the smaller grip frame in the first place, but he said he needed the longer gripping surface to help him control the pistol.
I also agree that if I were to be interested in a Kahr, I'd choose one of the metal-framed .40 S&W models, too.
