This roller-coaster spinoff to Forgetting Sarah Marshall often feels as if it'll jump the tracks and smash to the ground in a thousand pieces of WTF. It's a complete and utter mess from the big-loud-dumb start to the awwww-that's-so-sweet finish. And it's less a narrative than a loosely stitched-together hodgepodge of scenes starring the same characters as they hurl toward the titular venue, but not before making myriad soused and sentimental pit stops along the way in London, New York and Las Vegas — all in three days' time, tops. Which isn't to suggest that it isn't entertaining — far from it. It's quite the amiable mess, and occasionally it's uproarious, mostly due to Russell Brand reprising his role as Aldous Snow, frontman for Infant Sorrow, a sort of Spinal Tap redux. Jonah Hill plays the lower-rung record-label lackey who pitches an Infant Sorrow comeback concert to his boss, Sergio (Sean Combs), and is charged with getting an off-the-wagon Aldous to the show. Hill isn't reprising his role from Forgetting Sarah Marshall — the Hawaiian-resort waiter with the creepy crush on Aldous — but is playing a tempered version of said role. Judd Apatow produced — can't you smell the man-on-man love affair?