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The Martial Arts Pack comes with a Wing Chun dummy in your apartment that you can interact with (Jackie Chan fans should recognize it); the Shaolin Warrior outfit, which gives you increased melee damage; and the Shaolin Showdown mission that lets you fight monks whenever you please. I wouldn't recommend the DLC unless it becomes super cheap again and you want a quick way to fight a bunch of tougher-than-average dudes for an indefinite period of time. I stopped after killing 70 of them.
Please excuse my poor combat performance; it had been a while since I last played the game!