It’s getting hard to tell whether zombies or vampires are more prevalent in movies nowadays—both forms of nightstalkers are getting too numerous to count—but there’s always room for another good bloodletting. And “Zombieland” fits the bill. Basically it’s nothing more than a Troma shlockfest with a bigger budget, but it boasts an amusing script, a quick tempo and—most important of all—a cast that milks the madness for all it’s worth. And if, as often happens nowadays, the last reel proves somewhat of a letdown, that should come as no surprise.

In fact, there aren’t many surprises at all in the script, which is just a road movie set in an America where most of the citizenry have been turned into flesh-eating zombies by some unexplained infection. But there a few souls struggling to survive. The first we meet is a nerdy, hopelessly virginal college kid (Jesse Eisenberg) who tells of his first encounter with a zombie (as we’re shown in flashback, she was a beautiful gal from down the hall to whom he gave shelter hoping for some romantic payback, only to find her trying to devour him instead), and instructs us about the “survival rules” he’s followed ever since. Before long, though, he links up with the zanily macho Tallahassee (Woody Harrelson), who rechristens him Columbus, after the Ohio city he’s heading to.

On a grocery-store run where they off some zombies in pursuit of the Twinkie Tallahassee lists after, they encounter two girls—Wichita (Emma Stone) and Little Rock (Abigail Breslin), who relieve them of their weapons and SUV. But soon they’ve become a quartet driving westward to reach an amusement park outside L.A. which is supposedly zombie-free but is really just where Wichita wants to give her younger sister one last fling. (The whole idea is basically a shlock reworking of “National Lampoon’s Vacation.”)

But before the group arrive at the park, they stop in Hollywood for a night at the mansion of one of Tallahassee’s favorite stars, Bill Murray. And he shows up for a cameo in what’s easily the most inspired sequence in the whole picture. Unfortunately, after it the big zombie-killing finale at the amusement park turns out to be a dull comic-action set-piece, filled with lots of exploding bodies and gushes of gore. It’s all intended in fun, of course (as is the similar surfeit of black vomit and zombie-slicing in the first half-hour), but despite a stab at imaginative staging, it comes off as entirely too much of the same.

Still, overall “Zombieland” is a fun ride, not so much because the script has all that much to offer or Ruben Fleischer’s direction is so vibrant, but because of the cast, especially the men. Harrelson is the sparkplug, playing to the rafters with a send-up of macho bravado it would be hard to imagine anybody else matching. (It makes you appreciate his post-production explanation for attacking a photographer with the remark that obviously he took him for a zombie.) And he’s well paired with Eisenberg, who uses his jittery persona to convey a perfect attitude of dorky incompetence (though in the finale, as is obviously foreordained, he comes into his own). After his dreary cameo in Jim Jarmusch’s awful “The Limits of Control,” Murray makes amends with his delightfully goofy turn here. Fleischer’s great contribution is to give all three free rein.

As to the females, they’re fine but on a lower level, with Stone maintaining a hard façade as a stern romantic interest for Eisenberg and Breslin getting across the younger sister’s vulnerability.

Most of the technical budget of “Zombieland” seems to have been expended on the splatter effects, which exhibit an over-the-top cheesiness that’s entirely appropriate to the material. But otherwise production designer Maher Ahmad, art director Austin Gorg and set designer Timothy D. O’Brien and decorator Gene Serdena seem to have enjoyed themselves in putting together not only Murray’s mansion, but the western-style truck stop our heroes trash on their way to California.

As a piece of lunacy “Zombieland” may not be inspired, but it’s a better place to visit than most real amusement parks.