Extremely anticipated — or dreaded by numerous comic book fans who didn’t think it could be pulled off — director Zack Snyder’s Watchmen comes more than two decades after the original comic book mini-series by writer Alan Moore and artist Dave Gibbons published over 12 issues and then collected as “one of or THE most celebrated graphic novel ever.”

Really now? What about Frank Miller’s The Dark Knight Returns? Or try the original Stan Lee/Jack Kirby Fantastic Four run from issues numbering in the mid-’40s through the late ’80s, collected in various Marvel Masterworks hardbound volumes? I could go on about that…but won’t.

While Watchmen certainly broke new ground in turning the superhero genre upside down through what was a very bleak tale when you get down to it, it is not the holy grail of comic book writing or art. Sorry.

But yes, it’s still a landmark achievement that’s taken on legendary status over the past 20-plus years. The new Warner Brothers’ film is also a remarkable work. Some Watchmen comics fans will complain to the high heavens about it because of things left out and, most of all, changes to the ending (where’s “The Squid”? they cry). Oh, just go away, will ya?

While Moore, who’s frankly something of an iconoclast nutjob, has divorced himself from the movie (he considered the story “unfilmable”) and would not have his name in the credits, Gibbons has had nothing but high praise for the way it turned out.

Without giving too much away, let’s say it’s a tale of real-world (not as much so as Christopher Nolan’s Batman films) superheroes, left- and right-wing politics, hidden agendas, conspiracies and massive doses of paranoia, all set in an alternative universe USA, circa 1985, where a familiar face from the late ’60s and early ’70s is still the President in his latest of multiple terms well beyond eight years.

Did Snyder have to leave out things from the mini-series/graphic novel? Sure. And a lot of that, by the way, will turn up in one amazing DVD release as either bonus material or one super-long “director’s cut.”

The changed ending is the most drastic deviation, but it’s true to the theme of the original story and mighty horrifying in its implications, playing off the U.S.-U.S.S.R. Cold War and nuclear fears. There’s plenty of rich style at work here too, coupled with dense layers of connected stories and characters that are also right in line with the comics as well.

The casting is dead-on, especially Jackie Earl Haley, who is simply remarkable as the strangely masked, utterly pyschotic Roschach. Imagine a Batman who has gone over the edge. He’s creepy, beyond-driven and yet so relentless, you want to see more of him, though he’s all over the film.

As the anything-but-funny Comedian, Jeffery Dean Morgan — best known until now for roles as the father on The CW’s Supernatural and a ghostly presence on ABC’s Grey’s Anatomy — nails his role as well, playing a fascistic jackass.

As Nite Owl (Mark II — we also meet his aging mentor, the original Nite Owl), Patrick Wilson is also a Batman-ish type, but more so on the gadget, toys and cool vehicle side. He’s also the closest thing to the more traditional boy scout-ish hero, and sadly ridiculed for that. His slow-burn relationship with looker Malin Akerman as the second-generation Silk Spectre is another piece of the film puzzle. Though mostly a voice tied to a CGI character, Billy Crudup captures the essence of god-like powered Dr. Manhattan and offers a sense of a man who’s no longer a man at all, his humanity nearly gone until…well, you’ll see. Less fully-formed until the film’s final acts is Matthew Goode as Ozymandias, “the world’s smartest man.”

The soundtrack — loaded with Dylan songs by Dylan himself and others — is effective, and the mostly New York in the ’80s setting is familiar yet also gritty and otherworldly at the same time. The film simply looks fantastic.

The movie is also over-the-top violent, literally blood-filled, though some may argue no more than the comic book source was. But damn, it’s mighty graphic (and Snyder overplays the slow-mo thing). Combined with non-sexual and very sexual nude scenes, this is not, in any way or form, a film for kids. Maybe mid-to-older teens, but not children. They won’t understand from the trailers showing blue-glow Manhattan and the cool-looking costume of Nite Owl, but this is not a kid’s movie, period. This isn’t a barely R-rating film — it’s R all the way, and parents should know this, even if the kidlets saw action figures on shelves in a store.

One also has to wonder how it will play with the mainstream audience who has never heard of Watchmen before this and doesn’t give a whit about Alan Moore or Dave Gibbons and their hailed and worshiped graphic novel.

Watchmen is not the fun and clever thrill ride of last year’s Iron Man, nor is it the psychological tapestry woven in The Dark Knight (whose violence, in contrast, was implied more than in your face).

Nonetheless, it’s a creative and faithful adaptation down to the voiceover narrative and much of the dialogue lifted right out of the comics. To its credit, Watchmen is a movie you need to see more than once to absorb everything. Even with the changes from the comic series, you’re still not left with a “happy ending.” It only seems that way. Maybe.