I’d say that photos does a different job than either iPhoto or Curator and each app has a place in my toolbox.
photos is a database. Like all databases it is used to store and retrieve information. In the case of photos, the information stored and retrieved is about digital photos. It uses a standard MySQL database to store information which makes the information very portable if you want to export it, etc. It also happily holds very large quantities of photos without a performance hit. It’s web-based and it can run and/or be accessed from a PC, Linux box or a Mac.
iPhoto is a tool for *using* your digital photos, not storing them. It isn’t designed to hold thousands of photos (as evidenced by the performance if you try) but it really shines with it’s ease of use for output to slideshows, print, creating photo books, etc. It also has some image manipulation functionality (red-eye reduction, convert to B&W).
Curator is hands down the best image browser I’ve used. It is fast, easy and capable. I wish it were still being developed.
I use all 3 tools. photos is where I store information about my digital photos so I can search and find them later. I use iPhoto to show slideshows. And I use Curator to browse the photos I just downloaded and delete the junk.
Hope this helps.