If you're a longtime Aperture user, Photos is definitely a step back. Or rather, it's the clear signal that says it's time to look for other professional photo pastures. I can't recommend Photos as a full-time replacement, although I can envision situations where it would work alongside Aperture, such as creating small libraries for sharing with clients who don't own Aperture (both iPhoto and Aperture can open a library after it's been converted, but edits don't sync).
And if you're new to the Mac, drawn to Apple's computers after experiencing the iPhone or iPad, Photos for OS X will be immediately familiar, and even improve on your photo experience with better editing tools and iCloud Photo Library syncing.
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