Got this with the Season Pass. I only recommend this if you're interested in challenges. It adds no new items/abilities or story elements.
There are four levels, each with 15 waves to beat. Waves range from easy to challenging. Additionally, there are wave-specific blue ribbon challenges, which give you optional things to do or not do. These range from easy to extremely challenging. Some of them are buggy. Like an enemy stuck in midair after I shoot them off the edge of the map, don't see them and 1 enemy remains when I otherwise completed the challenge in a specified time limit. Stuff like that annoys the heck out of me. Not just that, but if you miss a challenge, you cannot restart a wave, you cannot save your progress, turn off the game and continue where you left off. You must sit through all 15 waves. It's the same with blue ribbons. If you get one, you must complete all 15 waves for it to save.
Developers will do anything in order to claim, "lOoK hOw MaNy HoUrS aRe SpEnT iN-GaMe!"
Fortunately, you can play 15 waves again and miss blue ribbons you've already gotten and get new ribbons and it keeps the old ones and adds the new ones each time.
Bottom line, only get this if you're interested in the challenge or achievement hunt. If you just casually like BioShock, don't bother.