Klaus was the consummate host: very communicative before we arrived, with advise on getting into the city. He met us at the apartment and took time with very helpful suggestions for what to do and see not only in the immediate neighborhood, but in the city in general. He was available throughout our visit if we had questions, and not only set up taxi service for our return to the airport, but met us at the apartment and waited for the taxi with us.
The apartment location is absolutely fantastic: there is much to see in Gràcia in the daytime, and we went out walking every night as well and had dinner or drinks outside on one of the many plazas. We ate an incredible meal at La Pubilla Restaurant -- a 5 minute walk from the apartment. Vegetable markets, bakeries, gelati shops are everywhere. There is a huge food market, Mercat de la LLibertat, very close by. One night we stumbled upon an outdoor concert in a nearby plaza: a local band (I assume) playing a combination of American Pop and Catalan songs. Although the area has trendy shops and restaurants, and Gràcia is certainly a destination, it doesn't feel mobbed and it also has the feeling of a community as well -- there were always kids playing in the plazas with their families nearby.
Gràcia is pretty centrally located, so most of the major sights in Barcelona are easy to get to -- we mostly walked.
The apartment itself is wonderful: beautifully designed and comfortable. The description and photos are quite accurate, the apartment has a lot of character and appeal, as does the building within which it is located. Although the Gracia area is quite active, the apartment is located on a quieter corner, so late night revelers are not a problem.