I just logged in to Photos to see if it would serve as a replacement (I use and like Picasa), and was I unpleasantly surprised! TLDR: A half-arsed clone that misses all the good in Picasa.
My first reaction: material design is great and all, but a clean interface that is undiscoverable (or requires five clicks to get anywhere) is useless. I see my Picasa albums on the home screen, except they show a date instead of the album name (I get the idea, it is just a timeline of photos ignoring my organization). Because a date is so much more informative. Also, there is this thing called Collections - because they added some abominations called Stories and Movies, which are also types of Collections (?). Except that Stories and Movies don't show up in my list of Collections, so why bother creating something called Collections in the first place?
When I go to collections, I see all my Albums (with names). Clicking on one takes me to the page with all photos.The map is gone (I like seeing all the places I've been on the home page of picasa).
They do have something new called Stories, and something called Movies. They both look like slideshows, except Movies is a YouTube video, while Stories is a interactive slideshow with some map integration to make it cool. Except I don't see how I can make my own story if I wanted, and the defaults are terrible.
All of this wouldn't be an issue if there wasn't one clear problem: Google is killing off Picasa. And why? To make way for Stories? It seems like an internal politics issue to me ("Look, I spent 2 months building this piece of shit, and I want it shipped and adopted, and I'll kill picasa if I damn well have to."). I get that some people might like the new features, and I can learn to live with the UI changes. Except that the best part about Picasa (and what was truly great about flickr) was the simplicity. They understood that there was a group that was interested in photography, creating and sharing albums, and that's it. And while you can still do most of that (I have no idea if the Picasa client will still work - that would be a deal breaker for me), we have to be subject to a bunch of crap just because someone wanted their pet project to get visibility.