I don't believe that one can compare anything written by Alan Moore to what ends up on the screen being ostensibly "based on the graphic novel". (The same applies to FROM HELL, which is another one most people pan, and one which I think is under-appreciated even though the style is breathtaking. I don't even want to think about the reaction that will ensue once THE WATCHMEN comes out!)
What seems to have been missed by most people is that this movie is about style as opposed to substance. It's based on a graphic novel. That's a fancy way of saying it's based on a comic book. On that level, the film succeeds admirably. The characters are archetypes of their literary forbears. They aren't supposed to be, strictly speaking, human. Of course the plot is grandiose, impractical, and over-the-top. Hello? Aren't most comic books like that? Good heavens, isn't most of STAR WARS?
I don't claim that this is a masterpiece. I do claim that's it's fun to watch if one approaches it with a willing suspension of disbelief. For a couple of bucks shelled out at the DVD rental shop, it takes one to a different world for close to two hours. On that level, it's worth a rental. It's also worth a rental, once one watches the movie, to listen to the commentary from various actors and to realize just how well these so-called "unknowns" do assorted accents that aren't even close to their own. Plus the golfing anecdotes are amusing. (And I don't even like golfing.)
This ain't CASABLANCA. Nor is it TITANIC, for which I eternally thank the gods. (Now, THERE was an overhyped piece of inaccurate trash that pretended to be history, but I digress.) But it's kind of fun, anyway, as long as one doesn't take it too seriously.