Mythic's Dark Age of Camelot has been live over four years now and has enjoyed its share of success and shortfalls. It's already paid for itself many times over and has a strong core following that pulls in enough coin to keep the streets of Camelot well paved. What keeps the game going after all these years is it's unique player vs player combat system (or realm vs realm) and the company keeps cranking out pve (player vs environment) content at the rate of about one retail expansion per year. Most of the expansions are must buys so players can keep the arms race going, so other than Catacombs (which added nothing to the game for most players), Dark Age of Camelot expansions usually do well. The latest commercial PvE add-on is Darkness Rising.