If you are a WWII history buff, you'll want to see this. But it's (in my opinion) primarily part of Germany's excellent effort to provide educational outings for high school students. The exhibits are good, with lots of photos, and some special exhibits from time to time. But I wouldn't say it's a must see, like most Concentration Camps.
Contrary to the previous poster, I've been to Cologne twice, for three days each, and I still haven't seen it all. This is a great city, with lots to see. (I'll never bother with the Chocolate Museum and the Cologne (scent) Store/Museum) Be ready to put in a full day. For example, better than the NSDC is the self-guided, ancient Jewish mikvah (ritual bath) near city hall. You leave your Passport as security, at a nearby big attraction, and they give you the key. (At least the two times I was in town.) This is a great city to visit. Consider the Museum Card instead of the card they push at the TI, if you like museums.