Like Chicago phenomenon Wesley Willis, singer-songwriter Daniel Johnston became a cult figure in the pop-music underground largely because of his mental illness. Raised by West Virginia fundamentalists and racked by manic depression, Johnston tirelessly promoted himself in the 80s with homemade cassettes whose romantic longing and demonic fantasy were embraced by alternative icons like Kurt Cobain and Sonic Youth. This engrossing documentary by Jeff Feuerzeig carefully distinguishes between the singer’s fans, who celebrate his illness, and his friends and family, who’ve had to live with his destructive behavior. But Johnston’s childish, repetitive tunes prove that he’s no Brian Wilson (or even Roky Erickson), which makes you wonder whether Feuerzeig is examining the singer’s exploitation or participating in it. PG-13, 109 min.