This DLC is essential to any CKII player. The addition of permanent armies, Retinues, makes army building much more fun, and the addition of extra flavor and events for the Byzantine Empire and Orthodox religion adds some extra fun stuff. The DLC is also extremely cheap (5 USD), and can be purchased on a sale for less than the price of a cup of coffee.
Overall, this DLC is essential, and I would recommend every CKII player who does not have it to consider investing the price of a cup of coffee into their game to greatly improve their experience.