The first impression is shock. A “typeface for extremely small sizes” shown in 48pt is not necessarily pretty. That is the nature of type specimens: we want to see details that aren’t meant to be seen in the real application.
Thomas Huot-Marchand, the designer, goes to extreme lengths to explain these details, but the first thing I did was to print a page from the PDF specimen provided. It works! Extremely readable at extremely small sizes, i.e. anything below 7pt.
Some of the tricks employed look familiar: the tops of stems bent backwards to open up joins and very explicit counters were measures I employed when I designed FF Meta, which was to be used at 7pt and below by the German Post Office. On a display face, these and other details would look mannered, but at small sizes, they become almost invisible. They make the sound rather than the tune of the page. There are several versions with less contrast, heavier serifs and wider tracking the smaller the intended size is.
Minuscule Deux which, presumably, is intended for 2pt type (!) has no curves left and no counters for some of the characters. The o, for example, is represented by its counter only: a black square. Strange, even weird, but physically successful in the intended size. To me, however, that looks just like blotchy rendering, but I wonder whether your ordinary reader would notice. Or, in fact, even bother to read 2pt type.
A radical departure, one worth considering both for its experimental character (no pun intended) and its potential practical uses. Typefaces originally designed for a particular purpose — from Century Schoolbook to Bell Centennial, from Frutiger to ITC Officina — have proved their worth before they become adopted by everybody for everything. This may well happen to Minuscule but it could take a while. For now, it is as strange as it is useful.