My family and I stayed for 7 weeks in this flat, and we found there's a lot to love about it: excellent location (near main spots, but not "in the mess" of mass tourism), plenty of shops, transportation and entertainment; spacious, impressively silent (double-glazed windows), full of character, accessible elevator, recently refurbished bathroom (with windows), plenty of equipment (dishwasher, drier and washing machine, blenders; microwave oven, etc).
Yet there is room for improvement: the flat is quite old and probably overused, which give it an overall "worn-out feeling" to the place; in addition it makes it hard to be cleaned properly (particularly the kitchen). Some pieces of furniture would need replacement (the wardrobe, sofas, the beds in the twin room, the kitchen furniture, for example), the oven makes a weird loud noise (with any function), the dishwasher doesn't clean properly, even if you rinse the plates first, so that we had to wash by hand all the "supposedly clean" dishes upon arrival and in many cases we had to re-wash our own after the machine failed to clean them. This said, Sergio is incredibly kind, caring and helpful, and he does an impressive job at speedily trying to fix anything you might point to him; yet there is a point beyond which "patching things" does not quite resolve the situation and a replacement would be needed - particularly for a flat at this price point. In other words, do not expect one of those sleek, design-oriented accommodations you may find on AirBnB (sometimes for comparable prices); more like you were staying in your old grandma's cosy flat; yet if you want a large, comfortable, well-located flat in the heart of Barcelona, this might be the right place for you and Sergi is a lovely host.