I saw this when it came out in the theaters and reread the GN going into the movie, and just recently Netflixed the "Tales of The Black Freighter" which was the comic within the comic. The DVD also included "Under the Hood", which was excerpts of the book that the orginal Nite-Owl wrote after revealing his identity, which also appeared in the GN between chapters.
I think that if you enjoyed the film and have read the GN, the DVD is a must rent since it really is part of the GN as a whole.
As for the movie I enjoyed it. I thought the choice of music worked, and techinally it was amazing. Snyder gave a film that was pretty true to the source material, which with comic readers is a big deal.
Have to agree that Malin Ackerman's Laurie was a weak spot, although in the GN you cleary see the awkwardness of Laurie and Dan's realtionship. But that doesn't excuse some of the delivery of her dialouge.
I think that many people went into this movie expecting alot more action, which is note really what this movie was. But if it gets people to read the GN, then more power to it.
Well if you can't tell, I recommend it as a rental for those who haven't seen it yet.