I know Flickr has been the biggest photo sharing website popular to photographers. I'm a casual photographer and I had to sign up for this when I took a course in digital photography. First, I'm not too happy about having to sign up with Yahoo just have to a Flickr account. It's like what Google pulls people to do with having a Gmail and a Google Account.
Second, the navigation is complicated and confusing, in my personal experience. It's really hard to navigate and the batch organizer looks really outdated. It's like you could go to the same page in two different ways for no reason. Deleting photographs or editing data on it is hard to find and also could be done in multiple obscure ways.
I'm deleting my account right after my photography course is over.