Covers are a tricky thing for any musician to handle. If done correctly, it makes you see the orginal song in a completely different light. The tone and cadence suddenly bring whole new meaning to the lyrics. If done poorly, it just makes you wish you were listening to the original song. Sometimes the covers completely outshine the original song in popularity, and sometimes they're songs that have been covered so many times that no one remembers how the original sounded.
Sonic Youth's cover of Superstar by Bonnie and Delaney still gets me. It's haunting and lonely and a little weird - fitting for a song from the perspective of a groupie. It's a much different take than the Carpenter's rendition, which feels much more like a standard torch song, beautiful and sad.