Of course, anything called “Genius” had better be smart. And this isn’t. One search turned up songs I can’t buy. A number of cover versions don’t list the original (e.g. Gnarls Barkley’s version of “Gone Daddy Gone” doesn’t link you to the original by Violent Femmes).
Overall, Genius seems to do a better job than the “you may also like” feature that used to be on the iTunes store (in my case, I’m sure that’s because it knows about all the music I bought from eMusic). I might even buy some music from its recommendations!