A new librarian
If you own an iPhone, iPad, or iPod touch with iOS 7 or iOS 8, Photos for OS X will be immediately familiar. The interface is patterned after the mobile version of Photos.
If, however, you've ignored Apple's devices lately, here's a quick overview of how you navigate the interface. Photos appear in chronological order, with the most recent images at the bottom of the list (and no option to sort in reverse chronological order). Views are broken out into a sort of "zooming" hierarchy that increases the size of the image thumbnails as you drill down: Years, Collections, Moments, and individual photos, which you switch using the unmarked arrows in the upper-left corner of the window or by clicking photos in the views.
Perhaps the best news is that Photos feels fast. Even on an older Mac (my main machine is still a 2010 MacBook Pro), Photos is speedy and responsive. The application switches to very low-resolution thumbnails when scrolling quickly, and photos pop when you view them solo. Photos seems to handle thumbnails internally better than its predecessors. (The first time you convert a library, you'll notice some lag as the initial thumbs are built.)
I didn't run into any serious problems converting iPhoto or Aperture libraries in my testing. In one case a library didn't have correct permissions and Photos offered to repair it for me during the conversion process. Large libraries can take a long time to convert, though, from hours to days depending on the size of the library and the speed of your Mac. (Regardless, make backups of your photo libraries!)
Faces and Places
You do need to be aware of some structural changes that occur in the transition. Notably, star ratings are converted to keywords, like "1 Star," and "2 Star." Photos now uses a binary Favorite label to mark photos you want to stand out (those Favorites will automatically copy to the Apple Watch). EXIF metadata is retained, even though not all of it is exposed in the Info window. Any custom metadata fields you created in Aperture are stripped away entirely. Events (iPhoto) and Projects (Aperture) are converted to regular albums.
The Faces feature also comes across, only in Photos it appears as a special album in the Albums view. Sorry, Aperture users who were able to turn the Faces feature off: it's on in Photos. However, it's not nearly the performance drag that it was in iPhoto, I'm happy to report.
In fact, Faces seems faster and more accurate in Photos, although the interface for assigning faces is a bit odd. Suggestions appear at the bottom of the window; you can double-click a face and type a person's name, or drag it to any existing faces you've identified above.
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