still playing with it, but my impression is if you don't need to do much to your images, Aperture alone would be fine. It gives you the basics of image manipulation without the need for Photoshop.
I'd say just from the organizational aspects, it is worth every penny. I've used a variety of programs - iView Multimedia, etc ... and none compare to Aperture from that aspect.
If you need Photoshop, you can always spring for it - or for that matter, Photoshop Elements. Even in my line of work, Elements would do 90 percent or more of what I need it to do.
Go for it, it will go nicely with the new iMac.