AtomPub was designed as a blog API, but with all the advancement the browsers have made as an application platform, it hasn't achieved wide spread use among bloggers. However, that doesn't limit the usefulness of it as a general publishing protocol.
My colleagues at Zepheira and I have been putting AtomPub into action recently. In our latest project we used AtomPub for a marketplace service registry. This allows others to publish the existence of their web service into collections. Because of the thorough documentation of AtomPub, it was easy to get everybody on board. With a simple example everybody started seeing the advantages of using a standard protocol. Generally speaking the AtomPub server was a breeze to maintain as all operations could be done using curl.
In the same project we also used OpenSearch with Atom to integrate multiple (private) search engines into a unified result. With some sugar and spice added to the "self" links we created a very impressive search solution.
Although AtomPub hasn't achieved much popularity within the blogging world, there are lots of benefits in using a standard protocol and AtomPub has proven itself as a great discovery protocol to us.