Sean Linnane did an excellent post on why the M1911A1 is such a popular pistol for those that trust their lives on it.
I agree with him - the pistol straight out of the box is a great platform to build upon - but the biggest selling point to me is the ease that it takes to modify it yourself. Anybody with half a brain can do basic modifications to the handgun to suit his/her purposes at their dining room table.
When I bought my Colt Officers Model back in '89, I took it to the range and put 500 rounds through it the first day. That night, I was installing new grips and dehorning it. A week later when the parts came in I installed a new trigger, springs, mainspring, and an extended slide release with the total cost still coming in at less than a fully modified clone would cost me.
22 years and well over 10,000 rounds later, the gun is still my favorite weapon.
For the original post over at Stormbringer, go here. He does a much better job of explaining his reasons for the love affair than I ever could.