Except that RAW processing and denoising in LightRoom 3 is so much better than in Aperture 3 for my Panasonic Lumix LX5 and Samsung GX-10. Do a cursory survey of the Aperture discussion boards, and you will see lots of people with RAW support that used to work fine, but broke after a Camera RAW update. Apple is also slow to add support for new cameras. Besides that, Aperture 3 is slow, even on recent machines with 4GB RAM.
I own both, and miss the "Places" and "Faces" features from Aperture in LightRoom. But given Aperture's deficiencies, I use LightRoom 95% of the time.
