If you haven't seen Dark City, take some advice from taiji_jian that I will pass along: Watch it on video, not in a theater, and kill the sound until a few seconds after you see Kiefer Sutherland appear. His opening voiceover spoils the entire film. By watching the movie without this voiceover, I was shielded from all the loss of wonder - not only from knowing the ending, but from being distracted when I should have been paying attention to the visual phenomena and ambiance.
The story opens ostensibly as a murder mystery of a most bizarre fashion. You might wonder if it is about a portal to a hellish world cf. Clive Barker's Hellraiser, or just an ambiguous (possibly self-fulfilling) hiccup in the universe as in Donnie Darko. In fact, it is something considerably more interesting and resonant: the journey of awakening and inner realization for Rufus Sewell. If that doesn't sound absorbing, well... just watch the film. It tells the story much better than any synopsis can, because it is so atmospheric that the setting reifies the plot (on several levels). I will say this: if Dark City were a puzzle game like Myst or Riven, it would start out looking like the most limiting world you could imagine... until you start using your imagination.
All in all, Dark City has barely two main actors (Sewell and Sutherland), and I honestly think it could be performed well on stage, but the special effects do lend something to the experience. The main value added by this film is how it faithfully reflects a fool's journey, in the sense of the Campbellian monomyth.
If you watch Dark City just for eerie visuals - occasionally disturbing, occasionally stunning, and always memorable - it will have been worth your time and the rental. Just remember that the ending, whether you love it or hate it, can stand alone from the movie, and that not all messages are about endings.
ETA, 05:10 CST Tue 11 Nov 2008 - I am remiss not to mention that triestine first recommended Dark City to me circa 2000, almost when it first came out (in 1998). As in many cases, it took me about six or seven years to heed her advice. Sometimes you have to repeat things enough for them to get through to me, and even then, I'll remember it as a buzzing from on high. Just to set the record straight, though, I do recall Mia mentioning this long before envoys arrived along the Via Paggia.