Before I begin, remember that Watchmen is a deconstruction of the superhero genre - taking stereotypical "superheroes" and finding their flaws. NOT a superhero movie. Do not go into cinemas expecting something along the lines of The Dark Knight, X-Men, Spiderman, Iron Man, etc. This is Watchmen - the greatest graphic novel-cum-movie of all time.
It's also a murder-mystery. It's a whodunnit!
The story premise (Spoiler-free) is this: ex-hero Edward Blake (aka The Comedian) is murdered by an unknown assailant. In this world, "heroes" (ordinary people in tights) have been outlawed. Uncompromising , violent, disturbed vigilante Rorschach, investigates. He believes someone's killing off former heroes, or "masks". He sets out to warn his ex-teammates, and discover who killed Edward Blake.
And he does, to great effect.
Sure, there's a lot more action than the book, but there has to be - it's a movie made for everyone, not just for Watchmen fans.
It stays true to the feel of the book - the characters are helpless, and only one of them has actual superpowers. Each and every character - and also the ending - are morally ambiguous.
After the dramatic conclusion, you are left wondering - without spoiling it - "Did the ends justify the means?".
This is a visually stunning, morally-objectionable, character-driven, dialogue-heavy, uncompromising, thinking-man's comic book movie.
If you have a chance at all, you should watch this.