Vivebarcelona has been fantastic, I've been staying with them for over 18months now!
The apartments have been very comfortable and locations AMAZING.
The service offered has been fantastic too, I've been dealing with Julien since the beginning and have always been taken care of, certain maintenance issues have been rectified very quickly.
I would strongly recommend ViveBarcelona's apartments, I personally don't think you can find better value for money in Barcelona, especially for expats and non fluent Spanish speaking people, they make life very simple for you.