Lightroom is Aperture done right, except for a few key things: Lightroom is free, easier to use, and only 11 megs.
Aperture is Apple's worst "pro" app yet. I've loved all the other pro apps, but found nothing special about Aperture. It can't do anything I can't do in Photoshop (apart from having a really cool zoom tool). Lightroom, on the other hand, will make any photo look amazing in a matter of a few easy steps (or hard ones, whatever your style)