My test for a movie like this is generally to ask my wife if she liked it. She knows nothing of the 1980's comic on which its based. She liked it, and was able to follow mostly what was going on. I wouldn't say she loved it but she was entertained. I was also entertained and I liked it, probably more than my wife did.The movie was a faithful recreation of the comic. I've only read the comic once (just last year) so I'm not a fan boy, but I don't think any included scenes were much different than the comic. Stuff was cut, otherwise, an already long movie, would have been interminable. For example, the whole pirate comic plot was removed. I can't say I'm sorry. The newspaper stand and the kid reading the comic make a token appearance.
The music added a nice dimension to the movie. There were two Bob Dylan songs ("All Along the Watchtower" done by Dylan and "Desolation Row" done by Chemical Romance), two Leonard Cohan songs ("Hallelujah", which you may more likely know by Jeff Buckley, and "First We Take Manhattan", both sung by Cohen), plus several pieces by Peter Glass which fit perfectly. And other stuff as well.
Please, please, don't take the kiddies to see this. In fact, I wouldn't let anyone under 15 see it. It has some pretty graphic violence (as does the original comic, but much more real here) and one sex scene (some hot goings on in the owl machine). But I'm probably pretty much out of touch with what 15 year olds watch so make up your own mind based on your own kids.
I've been struggling on how to wrap this up, to tell you how I really felt. I'm going to compare it to the Lord of the Rings movies. Before LOTR came out, I'd read the entire series 4 times, the 4th time just before the first movie came out. So even though I knew the story very well, I absolutely loved the movies. They made the story really come alive. I've only read The Watchmen comic once, but only a few months ago, so I remember the story pretty well. Here's where The Watchmen movie differs from LOTR. The Watchmen movie did not make the story come alive for me. It was as if I was reading the comic book on the big screen. Don't get me wrong, I think the movie is a stunning achievement in film making but it stops short of being great in telling the story.