(1)
Puh, difficult question. Rhythm is it! is an uplifting documentary, and Windows on Monday is pretty heavy stuff. I would say, it depends on your mood.

(2)
The rating at imdb probably seems poor, because it is difficult to connect emotinally to the film. It is very much shot like a documentary, with little dialogue. Could be too artsy, but then you shouldn't watch Windows on Monday at all. 
PS: I gave Lucy initially only a 58/100 ratingk, which is still good, but pretty low. Maybe the film isn't very successful as a film, but it has so many interesting elements and will probably leave you thinking for days about (german) society and human interactions. It challenges a lot of notions in a rather casual non-shocking way - this may also be a problem with audiences - but then you should have been there when Windows on Monday was screened. Probably 90% of the audience was clueless.