Well, obviously, OPML never caught on and became an important format. I can really only find two usages of OPML currently -- the OPML Editor (by Dave Winer) and Carbonfin's Outliner. Last time I looked at the OPML editor, it was somewhat difficult to setup and use (kind of expected for a GPL product). Carbonfin's Outliner only works on iPhone/iPad and may be using its own form of OPML internally. What I've been looking for from Carbonfin is for it's Outliner to implement import/export from Emacs Outline form since Emacs is implemented on most everything else and, even if it isn't, it's Outline form is much simpler and more useful than OPML.