We walked to the apartment from Gare du Nord, which took about 25 minutes. Sandra was there to meet us (although beware - "11 bis" is not the same as "11" - we initially tried to get into the wrong apartment building!) and made us feel immediately welcome and at home.
The apartment is in the perfect location: close to Montmatre in one direction and central Paris in the other. We had our 2 children (12 and 10 years old) with us and managed to see most of the sights on foot, although the Metro stations were really convenient when we needed them. There is a very good small supermarket just around the corner, and lovely pastries could be found within about 5 minutes walk in pretty much every direction. We didn't eat out in the area but we did buy decent take away chinese food from a local restaurant. The area is lively at night but, being high up, noise from the street was never a problem.
We had plenty of room for a family of four and all the comforts of home; the apartment was clean, everything worked well (shower, washing machine and dishwasher) and Sandra gave us the code for the Wifi which was very useful.
We would all highly recommend this apartment for your stay in Paris.