Great either for "historical value" or just because it's awesome.
Basically, the film created vampire films. It's got great use of shadows, a great villain, and its own historical value. Make sure you find a version with a good soundtrack. James Bernard (of HammerHorror fame wrote one, and the original (which is arranged on the Kino 2-DVD edition) is similarly good.
In this age of fetishized, romantic vampires, it's good to have a grotesque demonic bloodsucker like Orlok on whom to fall back. Through a combination of Murnau's camerawork and whoever did the makeup, Schreck's ultimate boogieman still retains his scares after all these years.