The airwaves are alive with the sound of cameos. Punch in any contemporary radio station and it's only a matter of time before you'll hear an R&B singer jutting into a rap hit, or vice versa. As marketing, it's genius: a way of luring more female listeners to rap and more men to plush R&B. As music, it's less of a sure thing. Ashanti's voice certainly adds a siren-like allure to Ja Rule's lithesome ''Always on Time,'' but the ploy has also led to all those dreary, revamped J. Lo hits.
Jay-Z and R. Kelly, two ambitious entrepreneurs who hooked up on last year's ''Fiesta,'' want to be the ones to take this trend to the next level. The Best of Both Worlds, their history-minded collaboration, expands the rough-meets-smooth concept to an entire album. From its cover art (where the title is designed in the style of a monolithic company logo) to its joint release by two labels, the disc is like a corporate merger.
When Jay-Z takes the lead, the album makes good on its potential. The electro-infused ''Shake Ya Body'' and the slinky ''Honey'' have that Jigga bounce and make Kelly's voice seem grittier than it is. Jay-Z has the best lines: ''No ex-boyfriend, no exit involved, just the highway exit that we exit off,'' he tells the lady he steals away in ''Somebody's Girl.'' But not even Kelly's disses of an easy target like Sisqo can compensate for the vanilla-flavored goo of the tracks on which he dominates. It's near impossible to listen to his string of smarmy lyrics in ''Naked'' and ''Take You Home With Me A.K.A. Body'' without thinking of a recent MADtv parody that brilliantly zinged the current sex-minors-and-videotape allegation (''You're my princess/I'm your prince/I'm taking one box of Thin Mints'').
''The Streets'' starts out promisingly, with Jay-Z playing a teenager in prison who tells his life story in flashback. Then Kelly takes over, and all the dramatic tension evaporates. So it goes for most of the album: Kelly slurps romantic come-ons as Jay-Z phones in kinky sex tales and swipes (at nemesis Nas, among others). It's as if they're acting out different parts of a psyche sensitive love man versus bad boy in each song. But ultimately, their skills feel worlds apart. C+