It is hard to write a review about Watchmen that will say anything new from the hundreds that have sprung into existence after the film came out, but hopefully I can provide a decent perspective! Let's say first that I have never read a graphic novel prior to this. In addition, I watched the film before reading the book, rather than the other way round, which might have affected my impression of one or the other.
Having given that caveat, I would say that this is a tour de force, a magnificent sprawling story that takes in many different ideas - including aging costumed heros; the idea of compromise; whether it is, indeed, a blessing to kill millions in order to save billions. The story is set against a backdrop of a fictional future, where Nixon has been elected for his fourth term, the Americans won in Vietnam with the help of Dr Manhattan, and the threat of nuclear war is ever present.
However, the plot itself - Rorschach has uncovered a possible conspiracy to take out costumed heros (not super heros, if you please - the only such is Dr Manhattan) - is pretty slight. The beauty of the book comes from the jumping back and forth in time to view the backgrounds of the main characters and how they became costumed heros; it comes from the many 'extras' such as snippets from Hollis' book and newspaper articles; and from the Tales of the Black Freighter that intertwine with many of the other plotlines.
The power of the tale comes from recognising the parodies of superheros e.g. Nite Owl representing Batman and his gadgets.
I also loved the use of frames to reflect events that had happened in the immediately prior frame e.g. the whole sequence with Dr Manhattan's television interview cutting away to the fight in the alley with Laurie and Dan. This took time to realise but helped to lend weight to a lot of the writing and pictures.
The whole tone was extremely bleak and the book felt very intense and somewhat exhausting to read.
But overall this was an amazing achievement and should be read by everyone who has even a passing interest in super heros, alternate history, graphic novels, ground breaking techniques. I loved it.