After I finished Deus Ex: Human Revolution, I played the DLC package, Missing Link. It turns out I should have played it before the second to last mission of the original, because the DLC mission takes place during the voyage there. It didn’t take away from the game, however, because it was completely orthogonal to the main plot and couldn’t influence it in any way.
In fact, the DLC mission starts you essentially at the beginning with no upgrades to the augments. Quickly, you get some Praxis and can upgrade as you see fit. I played the mission much as I played the main game: sneaking where necessary, but killing lots of enemies when shooting would work. The DLC would likely be played better as a stealth mission, and it would have been a good opportunity for me to practice the stealth skills, but I didn’t.
The mission is reasonably good, but sad in many ways. I won’t go into details, but many of your allies die, and the game forces you into a moral choice that requires some number of innocent deaths. There is apparently a trick to eliminating those repercussions, but once you know you need it, it’s too late. I presume they’re hoping for replay value, but I’m not going to bother.
Overall, if you can get Deus Ex: Human Revolution – Missing Link on sale, get it. I thought it was pretty good.