I have experienced the loss of a mag in a 172 just after takeoff. There were no indications during the run up or power check at the start of the take off roll that there was a problem. Once we got to about 300 feet AGL it let go. I thought the resulting vibration was so bad the prop would shear off so I reduced power and the vibration went away almost immediately. Despite the power reduction we were still climbing so I leveled off at 500 feet AGL and told the Tower I needed priority for the crossing runway. We landed safely 30 seconds later but I was pretty shaken by the whole thing. The main point I want to make is, you can do all the checks in the world, but that won't stop something important failing when you need it most, so be prepared!