Others may have a different impression -- but I think you can enjoy the game without having played the first one. The most important events from the previous game are explained to some extent in the early part of Darkness Within 2, so you do have a fairly good idea of what is going on.
If you haven't played the first game you won't understand the history of some of the characters as well, though.
Hmmm. The first one is a good game -- maybe play it first?