Critical opinion differed rather widely. The New York Sun described the production as, like its subject, a "misshapen and misbegotten waif," while The Philadelphia Inquirer wrote, "in comparison to so many well-made, well mannered operas that have crossed the stage of late, Grendel is a fearlessly R-rated pageant with so much to say and so many imaginative ways of saying it that you're willing to wait until the two-act piece finds it footing somewhere near the end of Act I."

There were cheers from audience and critics alike for bass-baritone Eric Owens, who, according to all reports, was excellent in the challenging and exhausting title role. The New York Times reported that the "stentorian bass got through it, summoning power when called for and inhabiting the part with abandon, even while wearing a body-encasing monster outfit." The paper added that the role of the Dragon, sung by mezzo-soprano Denyce Grave, "cruelly keeps her singing in a chesty, almost baritonal range, before pushing her to soprano highs."

The Newark Star Ledger said "the work has a compelling middle, but its long setup and anti-climatic finish are muddled and bland in a way that John Gardner's 1971 novel never is." The paper thought that musically, the "opera's tour-de-force, musically and theatrically, is the scene in which the omniscient Dragon grudgingly advises Grenel on the wherefores of the world."

Goldenthal's score won mixed reviews elsewhere. Newsday said the music "has its patches of generic modernist lurching, offset by considerable beauties"; the Inquirer thought that while Act I was unconvincing, "Act II stakes a claim for Goldenthal as a major composer with a powerful gift for dramatic specificity."

The Washington Post thought the wall was worth the effort, writing "visually, the results are magnificent — one stunning scene after another, and nothing out of place."