The P15-40 is a case in point: instead of firing seven .45 ACP cartridges before reloading, this one fires up to 15 .40 S&W cartridges. (Due to the bit of idiocy known as the Brady Law, I can only fire 11: 10 in the magazine, one in the chamber. Perhaps when that law expires, I can either legally convert my existing magazines to their full capacity or obtain new ones.) The grip is thicker than the M1911 to accommodate a double-stacked magazine. The gun feels better in my hand because of that, although that's intensely personal. Other changes include different levers for the slide stop and the safety. Of course, the barrel is different as well.
I haven't been able to fire mine since not long after I bought it. To disassemble the 1911, you push in a plug that holds in the recoil spring, then move a catch out of the way and release the spring tension. The last time I did that, my thumb slipped off of the spring and sent the plug flying. I wasn't able to find it, and the gun was rendered inoperable.
I got a new plug in today. It was too long (probably being made to fit the pistol's big brother, the P16-40), but five minutes with a Dremel tool fixed that. It appears to work just fine. I need to get hold of the folks who run the indoor range here in Fairmont and pay up for this year so I can check it out for sure. Assuming that everything works, it's going to Linucon with me for the Geeks with Guns event. I'll be interested to see what 1911 fan Eric Raymond thinks of it.