Helena's place is in a perfect location for shopping, eating, drinking, and sightseeing. It is less than a 10 minute walk to the beautiful Gran Via, and within 20-30 minutes walk to almost all major sights, including the Prado Museum, Puerta del Sol, Plaza Mayor, and Retiro Park. The nightlife in the area is great. It is very much like the Lower East Side of Manhattan - lots of small shops, stores, bars, and restaurants. There are grocery stores and pharmacies all over the area within walking distance, and the metro is about a 5 minute walk from the apartment.
Some cons: the electricity in the apartment is pretty weak and doesn't permit you to use multiple appliances at the same time. So, if you plan on doing laundry, baking, and using the stovetop at once, you will blow the power and need to reset the breakers, and then start your tasks over one-by-one. Also, the hot water only lasts for about 5 minutes, so long hot showers are out of the question.
Otherwise, we didn't have any problems and would probably recommend the place to anyone for a short stay. I stayed there almost a month and was generally happy, despite the hot water and power situation.
The location really is fantastic.