Spoilers

The Watchmen started out fantastic. The alternate history was both intricate and highly possible. Had a few things gone different, Nixon may well have been elected a third time, if not more. The history Alan Moore created wasn't a hack's alternate history, but a well thought-out, well-developed alternate world that made sense, everything fit together.
Further, the characters that drive the story are complex, full of their own demons, and are easy for the reader to identify with. It's a great premise that most everyone could relate to.
Unfortunately, the early momentum did not hang around for the entire graphic novel. The story started to drag when Dr. Manhattan and Laurie meet up on Mars. Adrian Veidt, who pushes the mystery of the story in the last third or so of the graphic novel, turns out to have a rather disappointing and surprising desire that motivates all of his actions. Finally, the "practical joke" that he pulls to bring about world peace is something straight out of a B science fiction flick that left me saying, "No. No. Just, no. Please, no." On the plus side, there is an ending that stays with you and will make you think hard about what you read.
So, despite its flaws, it's worth reading. However, while I understand it was a groundbreaking work as far as graphic novels, it's hard to say that this was one of the top 100 English language novels from 1923 to the present.
I'd give it a 3.5, if we had half stars. But we don't, and this is more of a 3 than a 4.