The location is outstanding, 5 mins walk in either direction from Republique and Chateau D'Eau metro stations which offer a choice of lines to go anywhere you need to for all the sights of Central Paris in a short time. Chateau D'Eau station always has some large groups of noisy young men hanging around it, so use Republique if this bothers you, but we had no problems there. It's also only 15 mins walk from Gare du Nord and 10 from Gare de L'Est, or one or two stops on the Metro.
The local area is rammed with bistros and brasseries as well as a range of non-French cuisines, as it's a very multicultural area. There's a great Lebanese place right opposite, plus a lovely bakery just two doors down for grabbing breakfast pastries. The street of the apartment is parallel to the huge Boulevard Magenta which is full of shops and restaurants and the area around Place de la Republique (5 mins) is also rammed with options.
If you like walking, you can stroll to Notre Dame in half an hour via the Pompidou Centre and most of the big sights are accessible in a similar time if you prefer to see the city on foot.
The space is basic but very well used. The flat has two double bedrooms with very comfy double beds, and there's a sofa bed in the lounge which converts to a third double really easily, plus there's another fold out bed in one of the bedrooms, so you can sleep a lot in the flat. The lounge has a dining table in front of the window and a small kitchenette attachment with basic cooking facilities, but is well stocked with utensils and crockery. The decor is a little basic but pleasant enough.
The flat is secure with keypad entry both from the street and the courtyard, and there's a lockable luggage cupboard downstairs, although when we visited the lock needed repairing.
Mathilde is super quick with her communication and sends a lot of useful info as soon as you've booked.
Overall, I highly recommend Mathilde's flat.