I have yet to explicitly see "express" boarding, but on a recent connection in Denver, the gate agent did ask for passengers to come up, check the size of their luggage against the cage and then check it if it was too big. She said this was due to passengers who've paid for their luggage to be stowed are getting upset due to people trying to get on with oversized bags.

Frankly, while I like the concept of express boarding, as it has been explained above, it seems to me that people could use it simply to get their luggage checked for free. Gate checking should be used for things that are required to get to the gate (wheelchairs, strollers, etc..) or carry-on sized baggage for which there's no room (all bins full or a regional jet, for example). Allowing a horde of passengers to gate check on a flight that could otherwise handle the bags seems contradictory to the policy of charging for checking luggage.