I stayed at this hotel for 4 nights in October 2004 and I really liked it. My room was huge and so was the bathroom. The hotel was in Avenue Louise, one of the main streets in Brussels and there was a tram stop just opposite the main entrance. The staff was friendly and helpful. I booked the hotel with Bancotel cheques and it was really cheap. I paid 50 euros per night for a double room. I'd stay there again.
The NH Hotel in Diegem is very convenient if you're visiting a site near the airport and still want to get into town to do the tourist thing. There is a train stop literally at the front door that will take you into the city (and avoid the 50+ Euro cab fair!). The NH brand is a little more upscale and has very comfortable rooms and amenities. By the way, there's very little to do in terms of night life out by the airport and you don't want to drink and drive, so the train makes it convenient to experience the Brussels night life and get back home safely. Likewise, if you're paying you're own way (and can't expense the costs) the train is much cheaper.
The best quality of the NH is the location - there's really not much else in terms of view, etc. Again, this hotel choice is aimed at the business traveller that needs to be in close proximity to the airport and still wants to have access to the center city.