iCloud Photo Library works with only one library that you designate as the System Photo Library. Also, if any of your photos are referenced — the files exist somewhere other than the Photos Library.photoslibrary package that is the default storage location — those images are not included in the iCloud Photo Library (but you can consolidate the library and bring the files in-house).
In addition to being able to view all of your photos on other devices, iCloud Photo Library transfers edits between devices. After you make a photo black-and-white on the Mac, for instance, it appears on your iPhone and iPad (and Apple TV and Apple Watch) the same.
You don't get the same level of editing detail between OS X and iOS versions of Photos, though. When you edit a shot on the Mac, the revised version appears on the iPhone, but editing that same photo on the phone reveals the sliders set back to zero. You can revert to the original version, but not tweak the existing settings.
It doesn't appear that Photos prioritizes syncing depending on what you're doing; if I edit a photo while other images are downloading in the background, the edits appear to be at the end (or near the end) of the queue. Again, we'll have to see how iCloud handles the influx of data, but at this point Apple needs to overcome the performance stigma that is commonly associated with iCloud.
I also often found myself craving more information. Is iCloud syncing stuck? Photos for OS X said I needed more space on my hard disk to store originals, but how much? I get that Apple wants to streamline the experience and not bombard users with numbers and technical details, but the alternative can be a state of anticipatory ennui — something will surely happen soon, but I don't know when.
Bottom line: Should you switch?
Photos for OS X is a free update that arrives with OS X Yosemite 10.10.3, so the question isn't whether to install it or not. Should you use it, or continue with iPhoto or Aperture for the time being?
By necessity, Photos for OS X is an application that arrives with a multitude of compromises. It needs to replace iPhoto for millions of people whose photo libraries are stored in Apple's default consumer photo application. It also technically replaces Aperture in the sense that Apple will no longer offer that professional app. And it has to also embrace the mobile reality that Apple itself created by making iPhones and iPads camera replacements for so many people.
If you're coming from iPhoto, Photos is definitely a step up. It's fast, it has improved editing tools, and even the loss of star ratings can be worked around (though I'd like to see them return).
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