I'm for the most part very happy with Photos.app. The majority of what I do with my photos is to view them, and Photos.app is perfect for that.

I wish it had better error messages. And that some of the album and album sharing was a little bit more elegant.

But other than that I'm very happy with it

When Photos.app just stops

When you import photos using Photos.app on OS X, keep in mind if you don't have enough space to import them all (even though it won't result in any extra photo due to APFS magic), it will just stop.