The original version of the song, released last year on Talk That Talk, clocks in at just 1:18. Here it gets bumped all the way up to 3:40, which may seem like a difficult task, but only if you're unaware of how awesome these two are at saying the same things over and over and over again. The gist of the original is that Rihanna would like some person to fuck her, and she imagines her body as some sort of birthday cake, even though it's not her birthday. It's a convenient metaphor, in that it allows her to say things like "Can't wait to lick the icing off" and "Can't wait to blow my candles out" and "I know you wanna bite this."
Chris Brown's first verse starts where Rihanna ends on the original, with the remarkably blunt line, "Ooh, I wanna fuck you right now," and right away you're reminded of how brazen this whole thing is, how genuinely clueless this guy seems to be about how his words will be taken, even if they're words that originally came out of her mouth. There are nicer, more subtle ways to tell someone you'd like to fuck them than by coming right out and saying it, and it's hard not to get stuck on the idea that he would be served well by a little subtlety, even if it goes against the tide of his chosen genre. He goes on to repeat mostly all the same lines she's already sung, adding a few new ones of little note. The bright spot here is that someone had the good sense not to let him anywhere near the "I'ma make you my bitch" part. One imagines it was neither of them, but some third party sitting behind the glass banging his or her head on the nearest desk.
UPDATE: There's a second Rihanna-Chris Brown collaboration making the rounds now, this one a remix of Brown's "Turn Up the Music." It's... I don't know. It's nothing, really—just a standard mix of platitudes about dancing and sex and whatever the fuck. There are some interesting conversations to be had about this reunion, but it doesn't seem like they'll have anything to do with the music itself.