All the people in the receptionist were so friendly and service-minded. Rooms were big, clean, with comfy beds. The spa + gym were a great luxury and good value (25 euro/person/stay), almost no people there in the evenings. Breakfast buffé (open 7-10) was great. A varied selection of scrambled eggs, veggies, fruits, bread, yoghurts, oatmeal, pickled herring (which was soo good!). They had soy-milk, big +. Coffee was really delicious too (and I'm a barista).
Overall me and my mother were delighted. It was an excellent stay. Location was also good, not in the city centre but just on the outskirts of it, but since Reykjavík isn't a big city, everything is within walking distance.
My only - would be that I requested a room towards the yard (since I didn't want any traffic noise) with a bathtub but when I arrived I had a room facing the traffic street with a shower. When I asked them about they said that only premium rooms (which was 40 euros more/night) faced the yard and had bathtubs. I feel like they could have answered this when I booked the room/sent them that request.
All was good though, the traffic noise didn't end up disturbing us.
I definitely recommend Centerhotel Midgardur!