Combining Will Smith's friendliness with St. Louis slang and a down-home drawl, Nelly has mastered his own brand of crossover appeal. On his fifth album, he mostly sticks to that pop-rap formula, cranking his distinctly melodic flow to hyper-speeds and playing the good-natured hedonist on cuts like "Party People." But when he tries to come off hard on a handful of Dirty South brawlers, he ends up sounding generic: "U Ain't Him" finds him rhyming about gunplay and warning no one in particular about "running up on the wrong nigga." Sixteen different guests help Knuckles through its changes of pace, with big names like Snoop Dogg, Fergie and Pharrell dropping by. And though these famous assistants may point to concerns about Nelly's declining market position, they also deliver some highlights: Check out the Chuck D.-assisted "Self Esteem," a Seventies funk-flavored jam where Nelly carps about racism, and hooky pleasures like "Body on Me," where Nelly and Akon compete to come up with the catchiest way to express how horny they are.