Apple released OS X 10.10.3 this week and with came a new Photos.app to replace iPhoto.app. It's more closely related to the current Photos app on iOS and I suspect it will be easier for them to add features that show up on all platforms.
So far so good. When you start it, it imports all your photos from iPhoto. It leaves iPhoto untouched so you can go back to it if you want. I turned on the Sidebar from the View menu. Photos adopts the iOS version's Years, Collections and Moments and gets rid of iPhoto's Events. Your old events are imported as Albums in a folder called iPhotos Events. There are still Albums and Smart Albums and (Shared) Photostreams.
Smart Albums are not as capable and several of mine got changed on import with "(modified)" appended to their name. There are no longer star ratings, they're changed to keywords like "1 Star" "2 Star" etc. That's fine, but Smart Albums now have limited boolean logic. You can list a few rules and choose if Any or All of them apply, but you can't add sublevels and nest (yes I tried holding down Option which is how you did it in iPhoto). So there's no way to convert what was "Keyword is Space and rating is greater than 3 stars" because you can't say (Space AND (4 Star OR 5 Star)). Fortunately that was my only Album that did that. Also Smart Albums can't have Key Photos, which is kind of ugly.
When Steve Jobs announced iPhoto Events he described them as a great automatic organizing tool. It collected photos from roughly a day together and I found it mostly worked (though the monthly photostream event was a little weird). So I named events and merged them when needed and it worked pretty well. Well now there aren't quite Events. If the idea is to edit them over time to get rid of them Apple didn't make it easy. For example. I went to a John Cleese talk and book signing and took several photos. I had just named the event John Cleese and in iPhoto I could search for that and find them. But if I don't want an Album (and Albums still seem a little more heavy weight than Events) then I need to rename the photos. While I can open the imported Event Album and select the seven photos, there doesn't seem to be a way to batch rename them. Or even to add the same description to each of them without doing it one by one. You can at least select several photos, bring up the Keywords panel and add/remove Keywords to all of them at once.
Also editing metadata is done via an Info popup panel, I'd prefer a side panel. And there doesn't seem to be an easy way to switch between the panel and the main window. So editing several photos not only involves a lot of repetition, it involves a lot of clicking between windows.
I did come up with one good productivity tip. Open the Keywords panel and assign number key shortcuts to the imported 1 Star, 2 Star, etc. keywords. So now you can bring up the Keywords panel and go through photos adding "stars" to them with one key.
I am not using iCloud for photos, I just store them locally on my iMac. My Photostream does bring recent photos to each device quickly, but I use iTunes to sync photos several albums onto my iPad and iPhone. The new iTunes only seems to support the new Photos app, you can't sync from iPhoto any more, so that's incentive to move. **Update:** Maybe that's not true, opening up iTunes again shows iPhoto as an option.
There is a nice map view that for me, shows wear and what I've eaten :)