This is hands down one of the most powerful shields in the game. It gets 2 charges (can negate 2 attacks) for each creature (your or your opponent's) that gets destroyed while it's active. Can stop an attack from a 500/500 creature with 1 charge, just as it would block the attack from a 1/1. This is the reason why it fails against swarm decks (FFQ is an obvious example).
But this isn't the only reason why this shield is the most loved: the AI tends to target this shield with any permanent control spell it gets (deflags/steals etc) so when this card is on your field, just pump it killing even your own creatures if necessary (otyughs and graveyards are a must) and enjoy seeing how the AI wastes 6 conflagrations/steals/whatever on your shield, that only loses 6 bones, not targeting your most important permanents. Sadly the evil AI is getting more and more intelligent, and now it can target (seldom) other permanents too. But this shield is great.
