It's both. It is a beefed up version of iPhoto in the sense that it have Faces and Places and Events but other than that it's a much much better editing and organising tool. I just bought Aperture 3 and I think it's totally worth the price. But that's me. It really depends on what you're going to be doing. Basically look at the features and think "Is that something I really need?" and then download the trial and test it out, if you like it then get it, if not don't.
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.
only been a mac user for 1 month. I am interested in slideshows and creating books of my kids. My mother is a professional photographer who uses CS4 and loves it but has never been a mac user either. I also want to get into editing some of my photos. I use a Nikon D5000 and Flip HD.
thanks