I actually avoid using them because the design is hard on the eyes and I find the way they present annotations to be annoying. I don't think there's anything special about them at all, especially when you consider that the idea is completely unoriginal.
But I'm not trying to use any lyrics site. Rap Genius garbage just always popped up on my (to me)-unrelated searches. People can respond to me with how they're not spam or they're not as spammy as other lyrics sites, but the point is that they pop up in the same frequency and same context as every other link-farm/content-mining crap-fest on the internet, like ehow, cha-cha, yahoo answers, mahalo, paidcontent, and so on.
Is anyone else getting more and more suspicious of the scorn that people have been gratuitously heaping on RG? Like, did they run over everyone's dog?