... While iPhoto is designed to work with one library at a time, Aperture lets you set up as many libraries as you want and switch between them instantly. And you can export a project — and all the related photos — as a new library. That makes it easy to do things like take a slideshow from your work computer to your home computer to finish it. Since the slideshow is a separate file, you can work with it directly — no need to import it into one of your home libraries. When you bring it back to your work computer, all the edits you made sync automatically...I gave up two years ago on multi-library support in iPhoto, support that would let me edit my photos while I travel and then merge them into my home library. The iPhoto 11 non-event, this conversion page, and Apple's $50 price point for iLife should crush anyone's residual hope for a better version of iPhoto.
So why am I not happy to buy Aperture 3? I have Aperture 2. The upgrade price is reasonable.
Because (shocking!!) .... Apple lies.
The iPhoto to Aperture 3 conversion is not seamless. Large amounts of metadata, such as album and event comments, image tags, and book definitions and the like are lost. Apple doesn't tell you this. That's because Apple is made up of Satan-worshiping sadists...