I liked 300 a whole lot more than I liked Watchmen. With 300, the plot is minimal and well-suited to goofy, over-the-top action scenes. Watchmen has subtler thematic material which clashes with the tone of the film.
The basic premise of the comic is: what if superheroes existed in the real world? What's the psychology behind their motivation and how would other people react to them? In this environment, such action scenes don't make sense. When Nite Owl and Silk Spectre fight off the gang, limbs are snapped to funny angles and spurt blood and knives are jammed into throats. It's jarring.
The worst is the sex scene. Following a spell of erectile dysfunction on the sofa, Nite Owl finally mounts Silk Spectre in the owl ship, swapping gratuitous poses for the camera as Leonard Cohen throatily warbles something wretched. It isn't sexy or sweet, it's mostly a little funny. There's a similarly jarring moment when the movie plays Sounds of Silence - the elements of soft, subtle humanity seem completely out of place in this kind of action movie.
For all the fidelity to the plot, there's not much fidelity to the tone of the comics. It is faithful, more or less, to the plot. Jackie Earle Haley and Jeffrey Dean Morgan are great as Rorschach and the Comedian, Billy Crudup does a passable HAL-9000 impression as Dr. Manhattan. It's not terrible, it's just not that good.
My recommendation - read the comic, it's better.
OK, I'm going to talk about the end scene now, so there will be some spoilers.
The comic largely sticks to a format of nine panels per page, arranged in a three by three grid. Frequently, a few of these panels will be fused together, but the regular pattern dominates the book. When the comic depicts the squid's devastation, it switches to one panel per page for six pages. You see bodies bloody and strewn about the streets. It's striking and forces the reader to confront the human reality of Veidt's plan. This is the climax of the story, and how the characters react to this event are important. The movie, previously quite happy to show bloodied body parts, glosses over the holocaust, showing only some property damage. The movie robs the story's climax of its emotional weight. This is intentional, apparently, because of 9/11 sensitivity. This sensitivity doesn't keep a gratuitous shot of the twin towers out of an earlier scene.
And of course I would have preferred the squid.