Cafe de l'Avenir is definitely what you want for a long stay in Paris if you have limited budget.
My girlfriend and I had the pleasure to stay in for 6 nights (28th Dec to 2nd Jan) and payed 450€ or so.
At the arrival, the staff offered us a welcome drink and brought our luggage to our room.
The structure of the hotel is modern and the rooms are tidy, clean and comfortable.
Our ikea-styled room offered a king-size bed, sink and shower. The TV is equipped with USB plug, so it is possible to watch movies or other stuff you have on a USB stick or even hard drive (not all codecs are supported, but whatever).
The toilet was shared but always clean and on the same floor of our room.
The hotel has free wifi, but i found the speed was low and sometimes the connection was unstable, but this might be due to my device (Android KK).
Ok for checking emails, wp and navigation. No boring registration needed :)
The recommended metro station is Garibaldi on line 13, direction St.Denis - Universitè. One access to the subway is right in front of the hotel.
In the nearby there is a McDonald's (open 7 days, until midnight), a grocery store (Monoprix) and some brasseries. The hotel has its own brasserie too! On sundays a pictoresque street market is hold on rue Garibaldi.
I would definitely recommend this hotel to anybody.