DLC adds eight missions in form of various challanges against different types of enemies or with different conditions (like timer) - which is nice, every one of them seems unique. It's a nice addition to the game if you liked it and would like to spend more time in it. Having a friend to try the endless co-op mode might be fun too. Plus, it's cheap and it gets discounted very often ;). The only downside is not being able to play new challanges in lower difficulty than normal.