Daniel Johnston is a talented, troubled, funny, frightening, exasperating, fascinating musician and artist. He emerged out of the 1980s underground music scene of Austin, Texas, as a kind of mascot and curiosity, and was championed by such underground luminaries as Kurt Cobain, the Butthole Surfers, and Sonic Youth. He is manic-depressive, which gives his art and life a great, child-like exuberance, but also a genuinely scary side.

This cult of Daniel Johnston – a justifiably respectful appreciation for his talent – is now threatening to become a popular religion, however. And as it does, his talents (if not his eccentricities) are being grossly exaggerated.

The trend has gained momentum in the last couple years. A recent covers album of his songs (“The Late Great Daniel Johnston: Discovered Covered”) included contributions from Beck, Tom Waits, Bright Eyes, the Flaming Lips, and Death Cab for Cutie. His art – crude magic marker drawings of curvaceous women, Marvel superheroes, the devil, eyeballs, and a boxer with the top of his head sawed off, most of them done on notebook paper – is filling galleries and selling for several thousand dollars apiece. But the most egregious example of the cult yet is Jeff Feuerzeig’s new documentary film, “The Devil and Daniel Johnston,” which opens today in New York and Los Angeles. It makes explicit what was long implicitly assumed among Mr. Johnston’s admirers: that he is one of the Western world’s great musicians and artists.

Convinced from an early age that he would become a great artist, Mr. Johnston began obsessively recording everything in his life: He made clever stop-motion home videos that parodied his family life; he recorded wandering and emotional sessions at the piano; and he surreptitiously taped knockdown drag-out fights with his mother over his unwillingness to do anything “productive” with himself (fights that he may have provoked expressly for documentary purposes). Mr. Feuerzeig’s film benefits greatly from this wealth of material. Along with recollections of a few close friends, performance footage, and tear-jerking interviews with his heartbroken parents, Mr.Johnston’s own recordings make up the bulk of the film.

And it is indeed bulky. At an hour and 54 minutes, the film assumes a deep and indulgent interest on the part of the viewer. Although well produced and often lovely, the film tediously chronicles every up and down of Mr. Johnston’s career and emotional health.

Much of it, especially early on, is interesting. As a teenager, he ran away (actually moped-ed away) with a carnival, leaving his distraught parents unaware of his whereabouts for several months. On the way home from a triumphant concert in Austin, Mr. Johnston grabs the controls of the prop plane his father is piloting and crashes it. As his delusions intensify, Mr. Johnston becomes convinced that Satan is manifest in everyone and everything he encounters. But the pattern soon becomes tiresome. He’s on his meds, off his meds, on them again. He’s in an asylum, back out.

What grates the most, however, is the relentless, outlandish praise for Mr. Johnston’s talents. Cobain famously called him the “greatest songwriter on earth,” a remark that says more about Cobain’s narrow taste than about Mr. Johnston’s talent. But everyone else in the film seems to agree.

Mr. Johnston is introduced by the master of ceremonies at a 2001 concert as “the best singer songwriter alive today,” and nobody bats an eye.”Imagine meeting Bob Dylan and he gives you his first six albums out of nowhere,” an Austin journalist says, describing his first encounter with the then-unknown singer. His homemade tape “Hi, How Are You?” is likened to “Meet the Beatles.” His music is said to be better than that of Robert Johnson and the Beach Boys, his art the equal of Marcel Duchamp’s. I kid you not.

The filmmakers get caught up in the game of reaching comparisons, too.The production notes liken one of his radio appearances to Orson Welles’s “War of the Worlds” broadcast, and the film places him squarely in the tradition of troubled geniuses that includes Vincent van Gogh and Sylvia Plath. In fact, Mr. Johnston’s the culmination of it.

It is possible for me to believe that there are people who prefer Mr. Johnston’s art to that of Mr. Dylan, the Beatles, the Beach Boys, Johnson, Plath, van Gogh, Duchamp, and Welles. His rawness, his obsessive longings, and his outsider status make him a compelling figure. I refuse to believe, however, that there are people who consider Mr. Johnston the equal of these others: To know them and appreciate their art is to understand that they are way out of his league.