I worked with ACR ever since I started working digital.
The problem I have with it is a faint blurry/waterlike effect arround some high contrast areas when I use fill light, with Aperture this is gone.
Also the color of Aperture and Leaf Capture are slightly better in the skintones, somehow ACR is a little reddish/magenta.
I can surpress this with a custom profile but still it's not up to par with Leaf Capture and Aperture (without profile).
I agree that the differences are very minor, all RAW developers can give good results in the end however it's what fits the look you want.
With Aperture or Leaf Capture I get a better result, but I think most people will be seeing no difference.
I switch to Aperture only a week ago so it can't be that I know that program more
The reason I switched to aperture had more to do with it's database options and in second place the lack of the artifact with fill light.
By the way the artifact only showed up on some shots, especially with high contrast and only on one side of the picture. I tried it on different machines and even took a sample with me to a tech on the pro-imaging in 2008, they saw the artifact and knew about it but did not know an answer, it appeared not often enough I think.
I'm not bashing ACR by the way, it's just that nowadays it's in the fine details and I decided that the combination of great dB structure and slightly better RAW control justified Aperture.