Particularly, those difficult images in which there is considerable dynamic range, Aperture just doesn't have the ability to handle the extreme adjustments necessary to create a quality image. Yes, the shadow slider is capable of lightening shadows, but it doesn't process them with clean results. And related to that, the well documented deficiency of noise reduction. In fact, I have been shocked at how well Camera Raw 8 is able to handle extreme dynamic range with excellent clean results.
That said, Aperture processes most images, that don't have extreme dynamic range or other problematic images, quite well. It's initial processing seems to always have a very good color rendition, at least for Nikon cameras. The mid-contrast control also works exceptionally well on some images. The same goes for the recovery slider. It works well as it doesn't create weird color shifts and works on images I thought could never be recovered.
Fortunately, I have found a decent procedure for "round-tripping" to Camera Raw/Photoshop to process some images. Thus, my workflow still is acceptable as I wait and wait for Apple to improve the algorithms in Aperture. Many of the sub-par tools, (noise reduction, sharpening, perspective control, etc) can be found in many of the plugins available, for a current solution. I also should mention I find the printing capability quite good, as well. But even that can be improved a bit since I found Qimage's detail resolution a tad bit better.
The bottom line and purpose of my comments is to let folks know that I wouldn't trash any images that Aperture can't process well. Options may include using another Raw processor, utilizing HDR software, or manually blending exposures in Photoshop. Depending on how demanding you may be about your images, you can do amazing adjustments to images through OnOne and Nik plugins without ever having to use Photoshop. Unfortunately, I demand the utmost clean artifact free images. Thus, many of my edits on challenging images include the use of luminosity masking, which can only be accomplished through Photoshop.