Stayed here for 3 nights in late August on the first leg of our 13 day trip to Iceland.
Rooms are quite small but if you don’t mind living largely out of your suitcase then you can survive well with the facilities provided. Everything is minimalist but perfectly adequate. I’m not so sure I liked the hand basin outside the bathroom but that was a relatively minor point.
Cleaning was to a high standard and very efficient.
Breakfast is included but using the fitness room, normally free to residents, was not - very disappointing!
The breakfast buffet was more than adequate with porridge and other cereals, breads, delicious flapjacks, a reasonable range of fruits and hot food as well as cold meats, cheese and eggs. However after 3 days of the same thing it was good to change diet.
We tried the Jorgensen restaurant on one evening but were disappointed with the quality of food.
As a result we frequented the wonderful Reykjavík Kitchen directly opposite the hotel and the Food Hall around the corner, in what was previously a bus station.

The hotel was very well situated for all major sites and bus tours. We arrived on Culture Night and the city was buzzing with huge numbers of tourists as well as events taking place and we were able to get to the heart of activities very quickly. We also were able to walk to the Perlan Museum in about 20 minutes.
Staff were helpful and very pleasant. We were so pleased that we were able to get into our room early on arrival.
Overall a good and comfortable place to stay to visit the city for a few days.

Let me start with the staff, which is, to this writer personal opinion, the most important asset any hotel has, and what an asset it is!
The staff at the CenterHotel Midgardur was kind, polite, friendly, helpful and attentive.

Examples.
We didn't like the first room we were offered and received a different one, immediately (much nicer view).
When the kitchen ran out of Mayonnaise, one of the staff brought us her own jar, just because she wanted us to have a perfect breakfast.

Adding that to the really perfect location, just in the heart of Reykjavik, a short walking distance from lots of shops, museums and other attractions, made this hotel a perfect spot for a vacation.