I've repaired 2 different windshields this way using the 3m kit. The sooner you do it after the crack happens the better the results tend to be. The key is to properly prepare it. Clean the windshield around the area completely with a lint free cloth and rubbing alcohol. Follow the directions exactly, and if your results are anything like mine you'll be hard pressed to know it was ever cracked. You can't tell even in the slightest that I've repaired mine.

Make sure you do this on a warm sunny, cloudless day. You have to do the repair in the shade, then take it ought into direct sunlight for the repair compound to cure from UV light.