Flickr is a great site. It's a repository, art gallery, and, in its commons section, a great place to locate royalty-free photography, the last being the best part. I suffer from a nervousness that I might violate copyright with all Web-based projects I do. Flickr and Creative Commons take a lot of the Angst out of the process and for this I'm grateful.
Insofar, as the third party features of Flickr, I'd have to wonder what the utility is. I appreciate the color pickr for its ability to match style concepts for Web design, and perhaps Mappr for the rare time when I'd like to dress up a map, but montagr just looks for fun than useful, which is okay, but not really why I'd look to Flickr.