I'm currently playing around with Dave Winer's OPML Editor, which he now uses for his blog. It looks quite fun and has a lot of features. My OPML Blog has collected some of the insights I've gained from it. I certainly won't switch over just like that - that would be Quark, which is not necessarily my target software. But it's fun to play with something completely different again.
Unfortunately, the OPML Editor has inherited some of the ailments of Radio Userland and Frontier, especially the handling of umlauts is not really smooth (I would like to have consistent UTF-8 support finally) and the runtime behavior is better than in Radio, but it still occasionally consumes too much CPU.
The concept of rendered outlines does have a certain charm. However, many parts of the rendering are not really accessible to normal users - you can edit the ancient table layout and make something else out of it, but the OPMLs are implemented with the internal OPML renderer and the HTML fragments are not so easy to change - and thus, for example, changing the language is quite cumbersome, as is the complete removal of layout tables.
More will certainly appear on the OPML blog from time to time, here I will write at most a few conclusions.