Excellent location, great for price - value. I know Madrid quite well, so I can totally say the location is unbeatable.
Do not expect something incredibly luxurious or stylish etc. Quite a hotel-like room, but convenient. It has all the equipment needed. (even extra plastic bags for trash, lots of glasses and stuff!) Even has a coffee machine, which is awww, cute.
Downsides: The studio is small. We were OK with it, but bear it in mind. The bed is not super comfy, the pillows were old, they turned yellow from white because of this. And they were quite uncomfortable... Maybe changing them could be a good idea. The sheets were not clean when we arrived but Sonia is very, very responsive, when we contacted her she immediately changed them. She literally responded within *minutes*. But these are minor things, really, and overall it didn't affect our stay negatively. The AC works pretty fine.
You get the keys from another place, which is OK I think, as it was only 3-4 min walk.
For this price, in the literal heart of Malasana, it's the best for sure. We didn't have any problem. We'll definitely choose this place again!