I often get asked whether I prefer Lightroom or Aperture. Having used Lightroom 2 (some), Lightroom 3 (a lot), Aperture 2 (a lot), and Aperture 3 (a little), I can semi-confidently share these anecdotal opinions:
Aperture usually has slightly nicer features than Lightroom, and I prefer much about Aperture’s interface, but it’s buggier and has more performance problems, even on very high-end hardware.
Aperture is always “improving” but never becomes great. I often felt that I was fighting it to get my work done.
Lightroom is consistently good and very stable. I’ve never felt that I was fighting it.
Your mileage may vary — but these were my experiences.