Slacker released its radio app for the iPhone yesterday, and while it'll have a hard time topping (which was ), there's a definite place for it.
Pandora's strength is the ability to create customized radio stations based on a particular artist or song--Pandora calls them "seeds." If the station's too narrow, you can add more variety by adding additional seeds, although this doesn't always work as I expect it to--for example, adding Yo La Tengo to my Pink Floyd station had no appreciable effect, still leaving me stuck in classic rock land (Zeppelin, Doors, John Lennon). If you don't even want to take this much trouble to customize a station, Pandora offers dozens of genre-specific stations, from Techno to Reggaeton.
The trouble with Pandora's genre-bound stations, though, is they're not customizable at all--you're stuck with somebody else's programming, and you can only skip six songs per station per hour (a concession to content owners, who want to sell you downloads and subscription services).
Slacker has a much better approach toward genre-specific stations: it lets you fine tune those stations along three spectra: personal favorites (play more or less of them), popularity, and year of release. (The Web version of Slacker is more like Pandora, with artist-specific stations.) This helped me create a lot of variety without having to muck about very much. For instance, when I fine-tuned Slacker's '80s Alternative station to play more "Fringe" songs, it did a nice job of mixing the obscure, like Flock of Seagulls' "Nightmares" (did you know they had more than one song?), with the weird but popular, like Peter Gabriel's "Big Time."
Both apps are free, so if you've got enough space, you can switch between them--Pandora when you're feeling finicky, Slacker when you just want somebody else to drive.