Shield blocking is a viable PvE combat skill that is calculated as follows:

Base chance to Block = 5% + (0.5% x Shield spec) + 3% per level of Mastery of Blocking.

It is further modified by your dexterity, such that kobolds block considerably more often than trolls. It only works in the forward arc, its is reduced by 50% if your opponent is dual weilding 2 weapons, and is somewhat reduced in RvR with the current group code penalties.

Large shields do substantial crush damage but none of the shield styles have bonuses to hit. Shields in keep defense are a bit more useful vs archers since the:

Base chance to block an arrow is: 30% + (0.5% X Shield spec) + 3% per level of Mastery of Blocking.

Engage ability substitutes all offensive actions of your character to get another 30% chance to block your opponent; in RvR this is close to useless. It is rumoured that shields can actually block enemy caster bolts… it happens so rarely that I cannot give you any numbers on the block rate.

Usefulness of Shields in RvR is moderate.