Disclaimer: Flickr Pro user since 2005. More than $25/year? Off to Dropbox.
Pro user since ~2005 here. I don't mind change in general, but I think the change they made here is definitely for the worse. Art needs space to breathe, and what they've done here is the equivalent of a photographic gallery plastering all the exhibited images in a wall-to-wall collage.
That sort of in-your-face layout may work for something like Facebook or Instagram, but not for something that is (was) meant to be for more serious photography.
I love the new UI and the new app.
The new site definitely looks more modern and glossy, but there's a reason museums don't display photos in a huge mosaic -- it makes it very hard to focus on and consider one image at a time. Now to browse someone's photos I need to go into the "one image at a time" viewer, which takes longer and leaves a long browser history.
Funnily enough, at the moment the details=1 layout even preserves the old "Did you know you can change the layout of this page?" link at the bottom, linking to a 404.