I've been to a few poetry slams, back when I was taking a poetry class at college. They were all right. Not really for me, since I don't perform and have difficulties concentrating on people talking. But the poetry itself was actually amazing. It's just that I would have preferred reading the poems.
Poetry slams are still good to have, since they're a way for new poets to break in, reading their work to a crowd and getting their work out there. Plus, some poetry just sounds better being heard instead of read.
It matters who's at the poetry slam. Both poets and the audience. Some that I went to the people cared more about how they looked while reading instead of their words, and the audience just gave applause instead of any constructive reviews at all. While other slams that I've been to, the poems read are great and the audience actually participates more with the poet.
Avatar credit: Fairy