I won't waste many words describing the look and feel of the new Photos app: Anyone who's played with the latest version of OS X should know what to expect here. That means a flat, two-dimensional design, and a thin, translucent menu bar that does its best to stay out of the way. The photos themselves have sort of an edge-to-edge effect, extending all the way to the bottom of the window, with no border or white space around them. Like other Yosemite apps, too, Photos borrows icons from iOS 8, right down to the "Share" button in the upper-right corner. Indeed, Photos for Mac looks a heckuva lot like Photos for iOS 8. Not a surprise, really: OS X Yosemite generally feels like it was inspired by iOS.
When it comes to organizing photos, you have a few options. By default, your pics are grouped by occasion ("Moments"), similar to the "Events" layout in the old iPhoto app. If you do a pinch gesture on your trackpad, though, you'll switch into a more chronological view, where you see your photos arranged not necessarily by event, but by the week, month or even year, depending on how far you zoom out. The effect makes for a gorgeous mosaic of tiny thumbnails -- all of them easily viewable by rolling over images with the cursor. Simply hold down the mouse, scrub it over a photo and watch it enlarge, just big enough for you to see what the picture is. Let go of your cursor and the image opens in its own window. Finally, you can view photos by location, using Apple's own Maps app, of course. I should add, too, that if you're upgrading from iPhoto, all of your old "Events" folders will be preserved, so you don't have to sort them all over again.
The editing tools are better this time around too. Not robust enough to take on Photoshop, mind you, but a clear improvement over iPhoto. At first glance, all you can see are the one-click tools -- things like auto-enhance, rotate, crop and filters. Speaking of filters, this isn't technically the first time Apple's photo editor has had something like this, but let's just say this is the first time Apple has gone all Instagram on us. The filters here are more extensive and nuanced than in the last generation of the software, with options that include "Instant" (a faded-out 1970s vibe) and "Noir" (one of several black-and-white options). As for the cropping tool, there's also a straightening wheel -- a handy feature if your shot of the horizon wasn't perfectly level.
Dig into the "adjustments" menu (the fourth icon from the bottom on the main edit screen), and you'll find some more advanced options. At first glance, all you'll see are sliders controlling light, color and black-and-white. Using a drop-down arrow, though, you can see additional sliders for all the factors that go into that final calculation (with light, for instance, you actually get control over exposure, highlights, shadows, brightness, contrast and black point). If all you wanted to do was adjust the master "light" slider, you'd still see the values for all six subcategories change dynamically. If nothing else, you could use this feature to better educate yourself on how exposure and highlights play off each other. If you were more advanced, though, you could also adjust each of those sliders individually. Make a mistake? Just double-click any of the sliders to return to the default value. Even better: There's a "Revert to Original" button sitting in the upper-right corner of the edit screen.
As I mentioned before, so long as you're signed in with the same Apple ID on all your devices, all your pictures will end up in the same iCloud Photo Library. Which means, any edits you make in the Mac Photos app will almost instantly appear in the iOS Photos app (and vice versa). At any time, too, you can press "M" on your Mac's keyboard to briefly see how the original looks, without actually reverting to it. When you're done editing, you can share to all the usual places, including Apple's own Mail and Messages apps, as well as AirDrop, Twitter, Facebook and Flickr. Don't see your photo-sharing service of choice? Well, Photos is at least "extensible," just like every other stock app in Yosemite, which is to say that developers can choose to give OS X apps a built-in sharing option, even if it's not native to the OS, per se.
Finally, Apple also made some changes to Photos' "Projects" tab, which is where you can see physical things you've been working on -- things like photo books, calendars and cards. With this version, Apple added a few new offerings, including a panoramic photo print. In addition, the company made it a little easier to lay out potential photo books, with a dynamic UI that adjusts immediately as you drag photos around the screen (think: photos that resize depending on whether you spread it across two pages of a book, or squeeze it onto a quarter-page square).
It's all very impressive (and pretty), and generally a welcome upgrade from the aging iPhoto. For some people, the editing tools won't be enough to replace Photoshop, much less Lightroom, but for everyday users, it should fit the bill, and be easy to use, too. And hey, who can argue with a free app?