We have really enjoyed Nathalie & Claudio's place, it's about 25 minutes away from the city center and 20 minutes from Brussel's Midi Station, by foot. You'll find plenty of little markets and shops in the neighborhood and even a pharmacy close by. It's pretty quiet too, we didn't heard a sound at night.
Their place is really easy to reach, except for the staircase to the apartment, which is in renovation and it's going to be a bit tricky to get your luggage all the way up only if they're really, really big. Check in and check out went really smoothly, their friend Mateusz was great, very helpful and available/flexible with our timetable.
The apartment is great, you'll find everything you need in the kitchen, from equipment to coffee and spices. They have a nice piano in the living room, I really wish I knew how to play it...You can adjust the heating in both the living room and the bedroom to your desired temperature, which is great and you get a pretty nice view from the windows.
If you plan to visit Brussels for a few days and want a nice, affordable and not too fancy place I recommend Nathalie and Claudio's place!