Genre doesn't get me see a particular film, but it puts it on my radar. MOON, DISTRICT 9, and SKYLINE were all films that I started to pay attention to because they were SF. But I didn't see the last one on the list, because...
Reviews are a pretty strong motivator, especially of reviewers that I like (I pay attention to Roger Ebert, Dana Stevens, David Edelstein, and the staff at the AV Club, as well a the CHUD website). The reviews for SKYLINE were terrible, so that nixed it.
As for the rest, I keep an eye out for various cast and crew that I enjoy. A writer like Aaron Sorkin will get me to see almost anything. A director like Duncan Jones (though, I admit, he has only directed one movie) will get me to see almost anything (I'm lukewarm when it comes to the trailers of SOURCE CODE, but I'll go because I like Jones and want him to be able to make MUTE). And certain actors appeal to me, like David Duchovny (for some odd reason) that cause me to seek out their films, even when the reviews are awful.
So, I guess it's a mix. But reviews are big, unless certain people are involved.