Aperture and Lightroom are very similar and you can achieve exactly the same goal with either piece of software. Aperture is a little better integrated into OSX than Lightroom is.
The real negative with Aperture is that Apple aren't as forthcoming as Adobe is about the software, with what is happening in future or updating it. I really think there needs to be an Aperture 4 soon, plus in my not even remotely extensive tests, I think Lightroom might be less resource heavy. I'm holding on to see if they do another Aperture update, if not, I think I may move to Lightroom.