isis was brought about by several companies getting together to make a standardised product that WAS NOT shimano compatible to help reduce the strangle hold that shimano had.
unfortunately when they increased the axle size to make it stiffer the bearing size had to be reduced to fit inside the shell of the b/b, this meant the bearing life was quite low. i heave known people kill them in less than 3 weeks!
the shimano equivalent, octalink, is better but still don't last as long as square taper and is compatible with only certain shimano chainsets.
it was generally thought that the oversize axle was better hence the move to external bearings.