The apartment was comfortable and we really had no major complaints. Albert (the official host) and Irene (who met us at the apt) were responsive. Albert was kind to let us check in early and Irene went the extra mile by waiting for us at the apartment when we arrived later than expected. The rooms were very small but all the beds were comfortable (2 twins, 1 full and 1 sleeper sofa). I expect this is typical in Madrid, if it is anything like New York City, it's probably very hard to find an affordable apt that isn't tiny... but I would not really recommend it for 6. I will mention we had issues with the hot water (the temp in the shower went from very hot to very cold and back again every couple minutes) but I am assuming it was a temporary problem that has now been corrected.
The kitchen wasn't well-equipped for cooking. The pots were very limited and the pans were all old teflon. The 3 things I suggested to Albert that were necessary but were not there at all: a usable cutting board, a can opener (there wasn't even a European style one) and a usable coffee maker. There was a tiny stovetop espresso pot, and 2 coffee makers that used pods, but no pods and no easy way to acquire them. I would suggest if they had a couple pods there to get us started, then at least we might have been able to find them at the store.
Finally I will mention if you are using public transportation, you can easily double the traveling times Albert listed in the description, e.g. to Las Ramblas and Plaza Espana. Walking would take even longer. Still, once you get the hang of it, the Barcelona metro (underground and buses) is great.