This was the perfect apartment for us in every way! Close to the market at Anton Martin for fresh produce, restaurants, sights, hints of Flamenco everywhere (main centre at A-M market with shops and bars around), loads of small shops - some old, some new...etc . It's very much a lively traditional neighbourhood and only 5-10 min walk to the Plaza Mayor and P d Sol and all the sights, activities. Easy enough to catch a taxi on one of the main roads if you are going further afield.
But, what was really so very special was the apartment itself. It's on a smaller street and so was very quiet. It was gorgeously warm and very personal - perfect for us family of four. Our teenage son slept in the loft (over the kitchen, a bit less private being next to the living/dining area) and our 10-year-old slept in the cosy attic room (which was confusingly on the lower level, to the right of the master bedroom but with sloped roof and porthole window).
It was lovely being inside, surrounded by lovely paintings/needlework and comforts of home - it was, honestly, hard to leave! Kitchen was fully equipped and had essentials, lots of gorgeous tableware, fabulous internet and tv. Special on fresh oranges at local supermarket, combined with electric squeezer meant fresh OJ every morning for an embarrassingly small cost compared to the UK. Fresh squid, octopus, clams, barnacles, etc, all in the market together with simple easygoing food stalls/restaurants.
We also received some info on restaurants, activities, sights and tours from Helena before arrival which was indeed very useful. Go and see a flamenco dance - both our kids enjoyed it tremendously and said it was one of their highlights. The museums were great - we bought a museum pass for EUR30 and saw all 3 museums. We did a Segway tour (HQ just around the corner). The touristy spots of Plaza Mayor and Mercato San Miguel were close enough to go to but far enough to avoid if we wanted.
Have trouble thinking of downsides of the apartment -- maybe the lower beams in the loft (my son hit his head in spite of the fact they were clearly marked). We would definitely return if it is available on our next trip to Madrid.