The Hollywood Reporter writes:

"Neighbors" represents a more real-world point of view than "Animal House" and "Old School," one that understands frat-boy excess not as a joyous manifestation of Bacchanalian life-force, but as a pointless, retrograde enterprise that should be stomped mercilessly — even if the middle-class banality that quashes it isn't everyone's ideal of adulthood. If a shirtless Zac Efron sells tickets, the sight of him being bested by proudly flabby Seth Rogen may sell just as many. Isn't it strange to see Seth Rogen becoming a model for the young American grown-up?

Variety praises Efron's "credible and intriguing" performance:

Lewder, weirder, louder, leaner, meaner and more winningly stupid than anything its director Nicholas Stoller and star Seth Rogen have ever been involved with before, frat comedy "Neighbors" boasts an almost oppressive volume of outrageous gags, and provided that audiences don't mind the lack of anything resembling a coherent story arc, its commercial potential ought to be enormous. 

HitFix says it's a new comedy classic:

"Neighbors" is a comedy people will return to often, and I would hope it leads to much more work for the writers. Stoller is already in demand, but I think this is going to make people reassess just how good he really is. What we saw tonight is technically a work-in-progress, but it felt very polished already. I may have a slightly sore throat as I head to bed tonight, but it's sore from laughing. One of the highest compliments I can pay to any film is to say I can't wait to see them again, and that is absolutely true of "Neighbors."

Film School Rejects laughed their heads off:

The physical comedy and sight gags are well choreographed, but the film doesn't lean too heavily on jackassery for its laughs. Nor does it treat the frat boys solely as adversaries, giving us scenes from Teddy and Pete's point of view to flesh them out (and "flesh" is definitely the right word). It would be easy in a comedy like this to do nothing but make fun of frat boys, but "Neighbors" doesn't take the easy way out.

The Playlist loves Rose Byrne:

Efron and Rogen are flanked by a ridiculously terrific supporting cast, including Ike Barinholtz as Rogen's best friend, Lisa Kudrow as the dean of the frat's college, "Submarine" star Craig Roberts as a pledge known as "Assjuice" and Christopher Mintz-Plasse as another fraternity brother (one whose, um, member is legendarily huge). But the breakthrough performance in the movie, and the one that took tonight's SXSW screening by storm, is Rose Byrne.