Own it, read it several times, and while I'm not a diehard fan of either, I'm not a fan of the movie, outside of the amazing opening sequences.  It's not just that so much from the book was cut and how much that changes the context, but the flick to me was just too slick and "badass" when the book really isn't.  I felt that the style hurt the a lot of what was going on there.

Case in point is Rorschach, the scene where the cops arrest him and they pull his mask off, to me, was always supposed to be this "holy sh1t, it was HIM?"  The dude we've been seeing pop up in the background all throughout the story turns out to be a major character and we never knew.  It didn't feel like that at all.  There's another scene earlier where the voiceover says "they didn't recognize me without my face."  it's this big wide shot in the book so you're thinking "he's here, which one is him?" but the way the movie shot it, as a pan, you know where he is immediately (although he is in shadow, somehow it just feels less dangerous when he could be any person on that crowded street).

Also, even though the movie borrowed most of the visuals from the book, like Sin City, they felt real different.  The book was straightforward and intentionally unimpressive, to counter the dramatic look seen in other books.  These were regular people in silly costumes, while the movie took those same images and made them look cool.

There was also the Sucker Punch issue of airplane toilet bowl colouring and slow-motion weightlessness, I never felt that anything was real because it was so stylized.  And all that stuff is real modern, so it never felt like the 80s (contrast that with the behind the scenes photos, where it really does look and feel like you're in the 80s).

I thought the cast was fine and everyone did a fine job, but I just didn't like it.