i upgraded my shimano M416 brakes to BB7s (also swapped the rotors!). there wasn't anything wrong with the M416s, as such, but adjusting them and keeping them tuned just right was a PITA (reaching through the wheels with an allen key). when the pads wore down i got the BB7s. the BB7s work just as well (after a few days to break them in) and keeping them adjusted in the sweet-spot is simple and easy.
for me the advantage of the BB7s isn't performance (although they're great), it's the ease of keeping them adjusted just right.