It's good. Makes a lot less sense if you haven't read the book, but a reasonable argument could be made that you don't deserve to enjoy the film if you haven't read the book.
(an equally reasonable argument could be made that, yes you do)
there's not much changed in the plot that much until near the end, some things happen in a slightly different order and the flashback structure doesn't flow as nicely as it did in the comic but... it's the comic put on screen. frames from the panels made into shots.
It's very violent, It deserves it's 18 rating for more than just Doctor Manhattan's big blue willy. Could the story have been told without so much of it? Probably, did this comic have this much? Yes. but there's something more horrific in film then on the page. Still, I'd rather this than a watered down '12' rated version... yuk!
one problems I had with Zack Snyder's take, I haven't seen his other films I haven't seen, uses speed ramps in the film leaving the impression that the heroes have superpowers or at least buffy style super strength.
It's closer to it's source material than 'V for Vendetta' but like that example it works better as a companion piece after you read the original than standing as a film in it's own right.