A buddy I grew up with bought a Ruger p91 back in about 1991, and he brought it with him when he came down from Colorado a few years later. We shot it some while he was here, and I became an instant fan of the cartridge.
After owning and shooting several different handguns chambered for it, I'm still a fan. To me, it's just the right combination of size, power, and capacity for a defensive carry gun.
But I'm really not sure the attraction is firearm dependent. I first shot that Ruger P91, then bought a Star "Firestar" in .40. After that I bought a Glock G27, and eventually traded an airedale pup for a CZ75b. I no longer own any of these, having fallen into the habit of swapping and selling as the urge tempts me, but I've kept a .40 of some sort pretty steadily over the last 15 years. My current is a Glock 23, and it's by far my favorite .40 that I've owned for all around carry use. I also shot a buddy's H&K P2000; it's a nice handgun, no doubt, but for my own use I prefer the G23.
I've yet to shoot a .40 S&W I didn't like, though.
Daryl