People are going to react very differently to “Watchmen,” the epic-sized adaptation by Zack Snyder (“300”) of the 12-part 1986-7 comic mini-series by genre guru Alan Moore and illustrator Dave Gibbons that was quickly reprinted as a graphic novel and, along with Frank Miller’s contemporaneous “The Dark Knight Returns,” marked a watershed in the medium. Various filmmakers have been trying for decades to bring it to the screen, but it proved a tough nut to crack, and now that Snyder’s managed to do so, the result is likely to divide the original’s devoted admirers while failing to satisfy those coming to it without prior acquaintance. (Moore himself, who put up with what he considered bastardizations of his later work in “V for Vendetta,” “From Hell” and especially “The League of Extraordinary Gentlemen,” disowned the whole project and demanded that his name be removed from it.)

Readers who’ve embraced the bleak, genre-bending vision of the book—about a group of retired (indeed, outlawed) costumed heroes in an “alternate” eighties America where Richard Nixon is in his fifth term and the world is on the brink of nuclear war, who reemerge to solve the murder of one of their own—actually have a good deal to celebrate here. Scripters David Haynter and Alex Tse have lifted much of the dialogue from the book, and Snyder is obviously in love with his source, using his formidable visual prowess (and access to a big budget and an army of technicians) to realize on screen the look of Gibbons’ panels. Just from those perspectives, if you’re a Moore acolyte even the 161 minutes lavished on “Watchmen” will seem too few.

And yet some of the book’s devotees are going to be dissatisfied. They’re likely to accept, grudgingly, most of the excisions and compressions, understanding that the material had to be whittled down if it was going to be a feature rather than a mini-series. But they’re going to be really peeved about major changes, especially in the ending. And they have a right to be. It seems simply incongruous for Snyder to have aimed so obsessively for the kind of visual fidelity to the book that fans would appreciate and approached the material almost as though it were Holy Writ for the most part, but then to have altered Moore’s plot in important ways. What’s the point?

Well, I suppose that the reason is obvious—he also wanted to appeal to a wider audience, which just might swallow the new concluding twist more easily than they would have Moore’s nuttier contrivance. (The change also made it easier for the effects team, but surely so crass a motive wouldn’t have swayed Snyder.)

But if that’s the motive, it’s unlikely to succeed, first of all because it’s improbable that a film that offers a political vision even darker and more dystopian that the oppressive dictatorship of “Vendetta” will appeal in today’s U.S.A. Snyder’s “Watchmen” seems more attuned to the zeitgeist of the Bush-Cheney years than to the opening of an Obama presidency. The timing is off. And by trying essentially to be a movie made for a bunch of fans while simultaneously appealing to a mass audience, it may well fail at both: whether it will expand the base or be confined to the one it began with (and disappoint some of it) is an open question.

That’s especially the case because if one approaches it cold, without reverence for the Moore-Gibbons original, it’s technically impressive, but also ponderous, self-important, unpleasantly violent and, quite frankly, kind of silly. Let me confess that I’m not a groupie. I have read “Watchmen,” but more out of a sense of duty than devotion, and while I appreciated Moore’s attempt to re-imagine the psychological underpinnings of the entire costumed-hero mythology, it didn’t quite come off—partially because the mystery at the heart of the plot was obvious, but also because the freshly-minted characters didn’t have the lived-in familiarity of the super-heroes who’d inhabited the comic pages for decades. They were pale imitations of the “real” thing. And frankly Moore’s political views represented—like those in “Vendetta”—a sort of radical chic that felt more like a pose than a deeply held belief.

These weaknesses are exacerbated in the film. The urban landscape is impressively created, but it’s the same sort of forbidding, rain-soaked, gloomy place we’ve seen in lots of earlier pictures. The “whodunit” part of the plot never grabs us, because the victim, the Comedian (Jeffrey Dean Morgan), is a brutish, vicious thug and the shamus who’s obsessed with tracking down the truth—Rorschach (Jackie Earle Haley), a vigilante who wears a white mask on which black blotches continually change shape—is as psychopathic as the villains he tracks down. (Sure, that’s the idea—but it makes him as difficult to connect with as the terrorist in “Vendetta.” And the reams of hard-boiled narration he has to deliver is the sort of overripe prose one can tolerate much more easily in print than as “drama.”)

Nor do the other characters generate much voltage, despite the fact that one—Dr. Manhattan (Billy Crudup), is an astronomically powerful turquoise-colored humanoid formed from one of those scientific accidents that recur in the genre. He’s initially partnered with the generically svelte Silk Sceptre (Malin Akerman), but she eventually turns to someone who can get intimate with her without dividing into several copies of himself as Manhattan does—the blandly boyish Nite Owl (Patrick Wilson), a sort of Batman Lite with a fancy flying ship among other tools of the crime-fighting trade. The final member of the group is Ozymandias (Matthew Goode), who humbly styles himself as the Smartest Man in the World and has used his brain to build a huge business empire.

In the time allotted to them within a single feature, these characters never fill out beyond sketch status, and as a result the parts don’t afford much opportunity for the actors to shine, especially since Snyder appears much more interested in posing them within his carefully-framed compositions than anything else. Haley fares best, simply because the combination of badass attitude and diminutive physique has a certain amusing quality. But Wilson and Akerman are ordinary, Goode overdoes the effete snob routine, and Morgan seems to be channeling J.K. Simmon’s Jonah Jameson at a far nastier level. Then there’s Crudup, a good actor so totally submerged in effects that he practically disappears. (His flashback autobiography is the best one in a movie filled with them, as the book was—though, true to tell, that isn’t saying much.)

To be fair, there are a few elements of the picture that do work unreservedly. The opening credit sequence is inventive and wondrously crafted, promising more than the film itself delivers. And it’s enjoyable to watch Robert Wisden do his Nixon imitation, though the cartoonish fake nose looks more like Cyrano or Bob Hope, as well as the briefer appearances by the likes of Henry Kissinger, Pat Buchanan, Ted Koppel, John McLaughlin and Eleanor Clift. But for the most part the picture has a solemn, ponderous, pretentious air that grows progressively wearisome over more than two-and-a-half hours—at least for one who doesn’t hold Moore’s book as the revelatory accomplishment many consider it to be.

So “Watchmen” turns out to be a $100 million cult movie that some fans may adore—if they can stomach the changes; but despite the visual virtuosity, it will bore many if not most of the uninitiated.