iPhoto's keywords ability can give you all sorts of sorting and organizing ability beyond the VERY EXTENSIVE abilities it already has. You'd be VERY surprised at what you can do in iPhoto with smart albums.

For example, let's say I have a huge pile of photos taken over many years with different digital cameras (as I think most people would).

How do I find, let's say, the ones I took with my old Nikon D70, with the flash on, between 2007 and 2009, in London? It would take me about 10 seconds to make a Smart Album with that criteria. And that's completely without keywords, just EXIF data.