If you are planning Paris for a few days or a business trip, this can suit you. Location is perfect and by metro or walk you are in 20-25mins basically at every important point of Paris :)
For the accommodation itself - nothing special. Very basic, super tiny apartment where larger man could barely fit himself into the bathroom or super short bed. Somehow, I have feeling that for the price you should get a bit more decent space than this. Accommodation in Paris for normal prices has been always quite a problem but in a different period I could have been in 4* star hotel close to Madeleine...
All in all, nothing was wrong but somehow it hurts me to pay 550 EUR for 3 nights for something like this...