I'd say that Onslaught and Predator are worth the price. Survivors is kinda meh but for the price of the overall three pack DLC along with the additional 15 achievements, I'd say it's worth the price. Although I would suggest you buy some of the DLC maps
because it can get boring playing on only three maps.
What Smile said is pretty accurate. Onslaught is like mercenaries mode, but more deadly and fast-paced. It's great. Predator mode is fun with a full lobby. It's amazing how quickly the Ustanak can go down with quality players.
One thing that isn't obvious right away is that to get your money's worth from the dlc modes it really helps to have all of the dlc maps as well. I would recommend them all, however.