Ella Hostel was a lovely place to stay for us. A great location for the metro and 'hop on, hop off' buses.
The shared front door was tricky to spot at first (and certainly doesn't fill you with hope as you approach it), but on the inside, the hostel is great!
The room we had was everything we needed for a base to see the city. It was clean and fresh, and the staff were very helpful.
The only real drawback is that it can be quite loud if you have other guests who don't realise the time they are getting in (but of course this is subjective to who is there at the time).
There are actually ear plugs at reception though, so if you are a light sleeper, they have you covered.
We would happily stay here again if we should ever come back to Barcelona.