I don't like any of Rihanna's ballads, Rehab included (thought a lot of people love it for some reason). It's a good beat and good lyrics wasted on weaker vocals. She does not sound good singing slow(er) songs at all, the only one I can stand is We Ride.

Now, when she's singing songs like SOS, Disturbia, and Don't Stop the Music...she's perfect, because then we have other things to concentrate on.