The room has three main assets: its excellent location; its quietness at night (the window opens on a backyard); the reliableness and friendliness of the host, François, who lives downstairs in the same building and was eager to help (it turned out that I had no real need for his help, as I knew the area already, but I appreciated his kindness all the same).
On the less positive side, it is a very tiny servant's room (so tiny that I actually suspect that, under present regulations in France, it couldn't be legally let on a permanent basis), there are no curtains or blinds to the skylight (it was all right on a grey December morning but in the summer months one would be waken up by the sun very early) and toilets are outside on the corridor. This was the main defect for me, to the extent that, had I been aware of it before, I would have looked for some other accommodation. Fortunately, I didn't have to get up at night but, if I had, it would have been quite unpleasant.
As for details, the breakfast advertised turned out to consist of a jar of Nescafe (without any sugar) and a kettle. But there was hot water for the shower and the wi-fi worked perfectly.
On the whole, I had a reasonably good experience, which I am unlikely to be tempted to repeat.