While on vacation with my partner, we made Karen's place our primary stay in Madrid for two weeks. And we were very glad to have done so. The place is much bigger than the photos make it seem and it provided the best general stay of our places with every amenity of the stay being just right. Transit was a short, peaceful walk away (note: coming back from the station, you'll have an incline to walk up, but it's not insane). Grocery stores, chinos, and Retiro Park are all close by (less than 10 minute walk to anything), which made our long stay easy and relaxing. The location was quiet and offered us a peaceful locale outside of the craziness and crowdedness of central Madrid, without taking us too far from it.
For longer stays, I can't recommend Karen's place enough. It really did feel like a home away from home and we're both missing it already.