Basically, this thing intensifies your spark. That means (theoretically) a more complete burn of the gas/air mixture in the combustion chamber; more power; and better gas mileage.
MSD is a big maker up here in the States. MSD 5s are no longer available here (they've been upgraded). If you are interested know that you also will need to change out a couple of other components to get this unit to fuction properly. But not a biggie. Installation is pretty easy.
Lee (68AutoBug) went with this sort of ignition system (although not an MSD) on his car and can probably give you more info on performance, etc.