Mainspring: I've had a number of those and didn't like them. Either they changed the feel of the grip, got in the way, or sliced up something in the process of drawing and firing. One concern is with the 1911 and the grip safety. The clip-draws I installed on my handguns didn't feel right and I wasn't comfortable with it. Might just be me.
However, it beats a soft loop for accessing a pistol. I think I would rather see a spring clip opening to the front attached to a belt loop to minimize bulk yet allow quick access without interfering with the feel of the grip. Just my 2 cents worth. Mikey.