iPhoto events and Aperture projects are converted to albums as neither of these items exist in Photos. Metadata including star ratings, flags, and color labels will be transformed into keywords and become searchable in Photos. Other IPTC metadata such as copyright, contact, and content data is retained as part of the image, but it's not visible within Photos. Custom metadata fields aren't transferred to Photos.
Those with OS X developer accounts will be able to play with Photos starting today when they download the beta of OS X 10.10.3. The rest of us will have to wait until the spring when the final (and free) version of 10.10.3 ships.
When that happens, Aperture and iPhoto will no longer be offered on the Mac App Store as new-to-you purchases. If you've purchased one of them in the past you'll be able to download another copy should something happen to the original — much as you can redownload an older version of GarageBand that's no longer being sold at the Store. For this reason, if you haven't purchased the latest iPhoto or Aperture and want them, act before they disappear.
I've had very little time with Photos but my general impression is that it hits a sweet spot for the casual-to-enthusiastic iOS and digital camera shooter. Its navigation is more nimble and, from what I can tell, its performance is significantly improved over iPhoto's, which I found sluggish with large image libraries. And, scaling back to the big picture, it's the first of the old iLife apps that shares a common experience among the Mac, iOS devices, and iCloud. All your photos, your most recent edits, wherever you are. It's an app worth looking forward to.
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