First thing to note is that in my last week of staying here, most things were cleaned, painted and repaired. So, you'll see the house in a better condition than when I arrived.
The two best parts are the location, which really couldn't be any better if you want to visit all parts of the city and that you get one separate bathroom per room.
That said, there are two big issues. Firstly, if you are staying in the summer and don't want to overheat and die in your sleep, you'll want to open the windows. However, the house is directly over a busy bar (and bar area) and you will be kept awake all night by both the people in the bar (until 2am) and drunk people walking home until sunrise.
Secondly, and more importantly, and I've never really thought about this with an airbnb property before, the windows are all barred to stop thieves. This means that the only entrance/exit is the stairwell, so if there is a fire blocking that exit, you will burn alive.
That said, the hosts are very nice people and the place is significantly less expensive than anywhere else in Barcelona if you stay for a month. Just maybe better to visit outside of the summer months and pray there won't be a fire.