Jason Snell writing for TidBITS:
The prospect of moving photography apps is daunting, but even die-hard users of iPhoto and Aperture would probably admit that the old versions had become increasingly slow and wonky. Rather than rip them apart, Apple decided to start fresh, which may sound familiar to users of many other Apple apps, including Final Cut Pro and both the iLife and iWork suites.
Jason's post is one of the better ones I've read on the upcoming Photos app. If you want to read one article to get a good overview, this is a good one.