Well, after reading the manual, it looks extremely promising and already usable, including to do complex things such as side notes or parallel, two-column bilingual texts.
Also, the frame system (to define a layout model for pages) looks both usable and powerful. Minor changes of the output aspect (fonts, page size, margins) seem easy to do.
Of course I would need to actually use it now, in order to have a more informed opinion and see whether the typesetting is actually good.
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