I use Aperture (AP). I have never really used Lightroom (LR). When I was on Windows, I was using Adobe’s Photo Shop Element (PSE) which I liked. I do miss some features of PSE on AP such as the timeline band.
I just saw this post on http://photofocus.com by Scott which I thought pretty much summed up why I am using Aperture. Nothing earth shattering, no flame post against LR, just simply preferred AP to LR.
Oh, I could have added that AP integrates better into the iLife suite than LR.