@Wessels - this first DLC is just basically you fight wave after wave of bad dudes. You have options to complete it with a specific challenge, if you do you earn a blue ribbon for that wave (e.g. kill all the enemies only using skyline strikes).
You earn silver eagles that you can use to upgrade weapons/buy vigors (you start out with 4, but you can buy the rest). You can also buy/unlock content in the "archaeological society," basically you're unlocking behind-the-scenes stuff.
It's ok, I guess, but I think the other one they haven't released yet is going to be better- that one takes place in Rapture and afaik has an actual storyline, instead of just killing lots of dudes. I might also be bitter because I got all the way to the last wave in the first arena and had to give up because the 2 handyman dudes kept handing me my ass, I died as I respawned, after a few dozen attempts and some different strategies I just said FUCK THIS and ragequit :/