I think it depends on what you use the application for. For the casual iPhone snapshooter, it is probably a great tool.
However I use a DSLR. I do not have it always with me, I carry it with me to certain "events" (a family reunion, a weekend trip, a summer vacation). I take a lot of pictures at one event, throw most of them away, and process the rest. All of that I do in Adobe Lightroom. iPhoto for me is just a nice tool for viewing and presenting the results, not more.
Even worse, I might have overlapping Events, e.g. a collection of pictures of my kids taken during summer, and another collection taken at a specific event, say, a birthday party. So the timeline does not work for me at all.
With iPhoto I could sort all these collections by date and it was very easy to find them. Now I have to sort them manually, or rename them from
"My grandmother celebrates her 95th birthday" to "2015-2-6 - My grandmother celebrates her 95th birthday"? Hm.