I am solidly in the rank amateur category of photographers, which is why I have never had reason to look into either Aperture or Lightroom. My camera doesn't even shoot in RAW.
However, James Duncan Davidson earns his way by shooting photographs and as such has used both Aperture and Lightroom (which is in beta). He compares and contrasts the two, though there is no clear 'winner' in this battle. Aperture is able to handle a huge volume of photos fairly well, but Lightroom's printing options are miles ahead of Aperture's.
[via Daring Fireball]