I use Aperture daily, along with photoshop and the other programs you'd expect a professional photographer to use heavily. Since the release of the 1.1 update several weeks ago, I can honestly say that Aperture is one of the nicer apps I use on a regular basis. But prior to that, Aperture was already saving me more time (read: $$) than any other tool I have.
Aperture is designed to let me import hundreds of photos from a shoot, in RAW, jpeg, whatever, QUICKLY add metadata, rate, sort, color correct for white balance, exposure etc.. This gets me to the point where I can now proof the images to my clients. The photos haven't been retouched, they are just in the form that lets a client see my skill as a photographer, and what images they have to choose from.
No matter who the client is, commercial, fashion, wedding, headshot... the faster I can let them see the proofs the better. From the 500 images in an average session... the client will only choose a few, which are then retouched in photoshop. I think this is what is hard for non-photographers to grasp; the sheer number of images NOT used. The workflow is designed to select only a few choice images, and then begin your post production processing of those selected images.
In many cases, especially with studio sessions, nothing really needs retouching after the image has been "tuned" in Aperture. Many times I'm sending the image versions directly from aperture to my lab printer. It is wonderful to use the Soft Proofing built-in to Aperture. It works great.
An important, but often overlooked core feature of Aperture is its top notch asset management system with versioning. Sure Subversion and CVS do version management better, but many of my colleagues have trouble with the concepts behind webmail, so Apertures simplicity in this area is admirable. I expect many new features will be added to the versioning and Vault system (like multiple library support), but much of what it does already is a major time saver. There are certainly alternatives, like lightroom, and bibble, which are each excellent in their own ways, but Aperture is more complete, and meets my needs better for now. Your mileage mat vary.
Lastly, I'm running Aperture on a G4 Powerbook. It runs fine. My RAW files are between 15Mb and 20Mb in size, and Aperture handles the hundreds of images per session fine. Could it be faster? Sure, what couldn't. But its not the nightmare that some report.