At first I was infatuated with A.C. Moore crafts, and it continues to have it's place for me among places to get crafting supplies. Primarily, it has a TERRIFIC yarn selection, especially if you like top-notch, fashionable yarns but are opposed to local yarn stores. (i'm not sure why some people don't like local yarn stores, but i know some don't... so this serves a purpose) AC moore also has good, wide-reaching, sales on their yarn -- unlike some other craft stores.
Other things that AC Moore offers that other places don't include: expanded candy supplies, dollhouse supplies, doll accessories, books, toys, etc. They do not carry as much seasonal merchandise as JoAnn's or Michaels, however.
The atmosphere in the White Marsh AC Moore is a little bleak. The lighting is almost too bright, and it feels more like Wal-mart than the other large craft chains. However, their prices are quite good and they typically have what you are looking for (if you know what that is)