I won't write a long review--I'm sure you can find plenty from better critics. But here's my quick thoughts.
It is a good movie. You have to work (a little) to keep up with it and ponder the themes. I'll give you a bit of a head start just so you can think about it a little before you go see it. The movie is set in 1985 in a version of history where the USA won the Vietnam War and Nixon was catapulted to popularity--elected to 3 terms. But the world is now more corrupt, more on edge. The Cold War is escalating, the Soviet Block is not crumbling. If you're over 30, then much of this may sound bizarre and intriguing. But if you're 20 ish then you might ask what all the fuss is about. The story definitely has a target age of relevance
The notion that this is a superhero movie is actually not much of a factor to the story. The Watchmen are just easier to pick out from other characters, but they contain all the same flaws (or worse) of the world they live in.
I liked this movie because it was interesting to consider all these 'what if" possibilities. The story asks some really hard questions and I found myself left wondering what my answers really would be. Any movie that keeps me thinking is good in my book.
BUT... this is not a happy movie. It is NOT a kid movie in any fashion. This is a nudge or two beyond simple violence and sex which it has plenty of. It may be particularly hard for women to watch. I don't mean that in a sexist "women can't hack it" way, but the movie's themes are quite sexist. The story's treatment of women is harsh. In my opinion this is over done, but there is no denying that the movie makes its point.
In the end (no spoilers), the movie leaves only questions. In true comic book style the story ends, but is not quite resolved. This movie will probably occupy your thoughts for a while. It might spark some heated spousal discussion. It is designed to provoke by forcing to light ugly situations and how one might deal with them. The only answers are gray.