The scuttlebutt is that the M Method increases your chances of hatching a shiny PKMN by 4 so that means, on average, your chances are 1/2048 rather than 1/8192.
I have hatched over 3000 eggs in pursuit of a legit shiny Magikarp. Once I get my Magikarp I am not likely to use the M Method again -- it's boring!! I much prefer chaining -- I've gotten over 300 shinies (54 different species) that way.
Good luck to all pursuing shinies!!
I agree that chaining is easier and faster, but what about those you can't chain? I for one have been trying to get a shiny staryu. My only method is either RNG abuse or Masuda method. =.=