Location, location, location! Three key words for the place :) We bought the T10 tickets as the guidebooks recommended, but we hardly even used the metro or bus (and we stayed in Barcelona for more than a week :)
When I booked the flat I was sure it was a whole apartment bus then Sid (the host) wrote to me and asked if I noticed it was the room and not an apartment (and I checked that option "whole apartment" while searching). First I was a little bit concerned but when we arrived at the place we realized that Sid is a really discreet guy and he uses a great "key code" to let you know if he's home or not :)