BioShock 2: Minerva's Den
Yeah! It's available. Cool.
In the same way that BioShock 2 was more BioShock, Minerva's Den is more BioShock 2 -- Rapture, plasmids, Big Daddy drill, Adam collection, Little Sister defense, blah, blah, blah. The mechanics are all here, anyway; the story is necessarily simplified, and while it is a little lacking in potency and memorable characters, I appreciate that it doesn't just try to one-up the original BioShock by making up another rival for Andrew Ryan.
It's hard to write much more about the DLC so far; the new levels look and feel a lot like your previous adventures in Rapture. There is a laser gun weapon, but it isn't powerful enough to be really cool; there's a new black-hole plasmid, but other than as a puzzle mechanic it doesn't seem useful. And with its lackluster story, the whole package is a bit underwhelming. But it does still scratch that tension/exploration/forensic/scavenger itch.
Progress: Just got to Operations