We had a very enjoyable stay at Javier's apartment. I've noted some pertinent details below:

Apartment
The unit is spacious and clean. The living room with a balcony overlooking the plaza is probably its best feature, although the large, well-equipped kitchen was also nice. The larger of the two bedrooms has lots of closet space and a spacious en-suite bathroom. The other is a bit smaller and only has a day-bed with trundle, so a little less than ideal for a second adult couple. The second bathroom is small but functional.

Location
A little off the beaten path -- as in, most cab drivers had no idea when I mentioned the name of the plaza and had to consult a map to navigate their way there (although it got easier once I was able to reference the main cross-streets) -- but that also meant that we felt like locals rather than tourists. Waking up Sunday morning to a free, live jazz concert taking place in the plaza below our balcony was a wonderful surprise. We also found a number of good restaurants and bars nearby. Highly recommend Estel de Gracia, just a couple blocks away, for a lovely dinner. Bobby Gin, just around the corner, specializes in Gin Tonics, and has a cool vibe.

Host
Javier was very responsive when I had questions or needs, but provided very little information to orient us to options in the neighborhood. This is not one of those situations where the host will point you to his favorite local haunts -- the only place he recommended by name turned out to be a chain. But as I said, he was professional and responsive. The only other thing to be aware of is that the cleaning fee is not included in the cost of the unit -- instead, Javier e-mails you the day before check-out to ask if you want to pay 40 Euros for cleaning or if you'll do it yourself. A little bit of a strange way to end the stay, but not the end of the world.