I agree with Rich and find most of the Concierge perks to be either meaningless or advertising hype. The priority boarding is only a separate line for CC cabins however very often the regular lines are shorter. They advertise the Concierge who is actually stationed on the ship but is available to all classes of cabins so that is no big deal. If you are Diamond in C@A you will get Elite on Celebrity which again is a separate line. If you do book make sure whoever you book with gets you a Captain Club number and it is associated with your reservation.
You should also be aware that Century is going into drydock early next year for a three week major overhaul. Because of this cabin classifications and numbers may change. The exact changes have not been announced but the drydock is scheduled for I believe Feb. and March prior to Alaska season.
Since it is so popular I believe Celebrity will be putting Aqua Class cabins on Century during the drydock as well as Blu restaurant which replaces the MDR for Aqua passengers. Prices are usually a little above concierge but are worth it.