Herein be spoilers, ye have been warned...
I can't judge this movie outside of my own experience of having read the graphic novel twice, and from that perspective, Watchmen was mostly boring. The production was so faithful that I knew what was coming up each and every scene, and while it was sometimes interesting to see how the the filmmakers would translate Alan Moore and Dave Gibbons' work to the screen, that source material was never vital or interesting enough to really capture my attention in the first place.
Watchmen is the Citizen Kane of modern comic books: it added artistic techniques and adult themes that matured the medium beyond what had hitherto been solely children's literature, but it has since been so copied that it's often hard to see the original work as anything other than quaint and tired. The truth is that Watchmen wasn't all that revolutionary, as the deconstruction of the superhero goes back to its earliest creation, when Philip Wylie created a superman in his novel Gladiator and then showed how that kind of power is ultimately rather powerless. Yet Alan Moore is a great writer and created the best comic book I've ever read... it just wasn't Watchmen, but rather his true masterpiece, From Hell.
So it's no surprise that the problems I had with the film are almost all down to the graphic novel itself. While at first I found the performances (particularly Patrick Wilson's Dan Dreiberg) clunky and uninvolved, the truth was that this was mostly due to Moore's bad case of Lucasitis when he wrote the prose-like dialogue in the comic. That the film takes so long to get started was also a failing of the comic. And that so few of the characters are likeable or interesting enough to empathize with is also inherent in their portrayal in the comic. Watchmen is a case study in why the anguish of fanboys should be ignored and filmmakers should first try to create their own good movie from comics rather than treating the source material like holy scripture that cannot be edited.
The film may actually be even worse than I found it, because, while I could easily keep track of all the various subplots and characters, and while I knew what emotions and ideas the film was trying to capture, all that was due to my having read the graphic novel. I wonder if someone who hadn't would able to do the same, and, when we get the shot of the pudgy costumed Nite-Owl screaming with over-emotion in the snow at Rorschach's blood spot, I can't help but feel the whole thing had descended into cheesy camp for the non-initiated. All that said, as the film neared its end, I began to feel it was turning out marginally alright. That Zack Snyder changed Moore's ending by replacing the giant alien squid to the threat of Doctor Manhattan himself actually felt like an improvement, but then they screwed it all up by leaving out the comic's most lasting note (for me).
In the end of Moore's Watchmen, Veidt implores Doctor Manhattan to reassure him that he did the right thing, that "it all works out in the end," but Manhattan simply leaves with the ominous parting words of "It never ends." It is a damning statement on the very nature of superheroes: the old-time heroes of the forties either faded away, their exploits having been little more than media-generated advertising, or turned into fascist nightmares like The Comedian; the later generation of heroes had become equally obsolete, serving as tools of a government that would later outlaw them, and finally are made irrelevant by the truly superpower of Doctor Manhattan; and Ozymandias, the last real human superhero, the "smartest man in the world", the only one who really understood how the world worked and the only one who was able to use his powers to actually change things, ultimately will likely also accomplish nothing, as Manhattan's final words suggest that human nature will one day take its course and mankind still likely awaits its own destruction. Watchmen's final message is that superheroes cannot save humanity from itself (these "heroes" are far too human to begin with, and are simply acting out their own "too human" neurotic fetishes). While it is clunkily delivered in the comic, it remains a compelling theme.
None of that is in the movie. The "nothing ever really ends" line is still there (spoken by Laurie to Dan in the next-to-final scene), but that it is not delivered by Manhattan to an apprehensive Veidt strips it of its thematic power. Watchmen the comic book may have its flaws, but it did try to be about something. Watchmen the film is, like all of Snyder's other work, absolutely meaningless - superficial and lovely in short doses, but with nearly three hours to examine it in detail, Watchmen is far too long to hide the fact that Snyder is a vapid storyteller.
I could go on about other things: the film is gratutiously more violent than the comic, Snyder's usual slo-mo camerawork destroys the pace in a film nearing three hours, all the Nixon scenes looked like Snyder was trying to rip-off Dr. Strangelove more than adapt Watchmen, but whatever. It should have been a rental.