If you have a dSLR and shoot RAW it is a must have. Otherwise, it is a welcome upgrade, but not a necessity. Basically Aperture let's you handle RAW photos in a much better way than iPhoto, and also gives you better tools to edit them.
One of the things I like the most about Aperture, besides what I mentioned above, is the organization system. Whatever is the way you like to keep your photos organized, you can easily do it within Aperture.
Do you find iPhoto limits you?