The answer lies in what functions of iPhoto are you trying to replace. If you want better image-enhancement, but not the learning curve of Photoshop, then get Photoshop Elements. You can use iPhoto to keep things organized and use Elements as the external editor.
If you need a real digital asset manager (DAM), the leaders are iView and Extensis Portfolio. I prefer iView; it comes in a "lite" version and a pro version. Release 3 is a very powerful application, and Extensis is releasing version 8 of Portfolio. Keep in mind, that both of these applications cost $200 (and you'll still want a real image editor like Photoshop or Elements).
The advantage to something like iView is that it can catalog all kinds of digital items, such as PDFs, font files, etc, and it can even catalog files from removable media.
I'm not a professional photographer, but I've been using Photoshop since day one. I was befuddled by iPhoto's "organization" scheme; I was expecting it to be like iTunes. Sadly, the Windows version of Photoshop Elements has a much better organizer than iPhoto; however, it is not as tightly integrated.
I use iPhoto for things like family photos, etc. I use iView for work-related image files. iPhoto is custom-made for things like that my family members are amazed when they ask me to post photos of some event, and I have it up on my .Mac site within minutes.
I have my own scheme for organizing work items, like Flash files, screen shots, PDFs, and other items. I can use iView to get quick snapshots of the files, organize them where I want to, and know I can quickly find them.
Aperture seems designed around a RAW image workflow, not general image files. A real DAM is a better investment if you need organization.