I stayed at Hostal Mare Nostrum before running the 2011 Barcelona Marathon. We were looking for inexpensive accommodations and we found them! The hotel was located right on Las Ramblas, with easy access to the metro and some shopping. It was a rather crazy area to be located in, but I can't complain too much. For what I spent, I think I got a lot - an extremely friendly and knowledgeable staff, coffee and continental breakfast every morning, and a clean (if sparse) room. I wouldn't stay here again due to location and minimal amenities, but for an inexpensive stay in Barcelona it was worth the money.

I can highly recommend the Mare Nostrum to anybody on a low budget looking for a friendly staff in the middle of a busy and exciting city. It's the third time in a row that I've been to the Mare Nostrum with a group of students and I will definitely return again! The location on the Ramblas next to the Opera House Liceu and the metro station is of course ideal, but it's also only a 10-minute busride to the beaches of Barceloneta.
In comparison to other low budget hotels in Barcelona, the rooms at the Mare Nostrum are clean and they all have reading lamps, which is not the case everywhere! The staff is multilingual (even Japanese!), very friendly, helpful and also professional in emergency situations. When one of my students fell ill in the middle of the night, they notified the paramedics, who only took 10 minutes to get to the hotel!
My favourite room in the hostal is the breakfast room with a large panorama window looking down on the Ramblas. When there are groups in the hotel, space can become a bit of a problem, so try to get there before the groups for a front row table overlooking the Ramblas. Thanks to more space along the wall, the breakfast buffet has improved , especially the toasted ham and cheese sandwich was a great hit with my students!
You will have to carry your suitcase up the narrow staircase to reception on the first floor, but from there on there's a lift going to the upper floors.
I will definitely return again!

I loved that place the people where very helpful going out of there way to help us in any way possible. May be the rooms are small but so well locatated we did not mind. Having our breakfast looking out on the Ramblas was a delight that I was not even expecting in a such low cost hostal.

Friend and I stayed here and got to experience 3 different rooms due to issues with the first one. The shower door was not containing the water from the shower so it was flooding into the room. Other than that the experience was fine overall. Location is what this has going for it. Super centrally located. Beach is an easy walk, as is the large shopping mall. You are right on a tube/metro stop so the whole city is at your disposal. Started by taking the Renfe from the airport to the Hostel and it worked without issue. The metro stop is Liceu on the green line.
Careful at night the prostitutes can get a bit grabby. That is the surroundings though and it's not overt nor did we ever feel unsafe day or night. This place is a Hostel not a Hotel so take yourself back to college and remember the dorm room situation. This is it, but with a light breakfast. The breakfast was great to have and nice to overlook the Ramblas while dining. Pretty much pastries, semi-cool milk and juice. Toaster and breads. Just enough to last us until lunch. If you are two people and need some space, I'd go with the 3 bed room setup so you'll have somewhere to put your luggage. Also the bathroom is nicer relatively speaking in those rooms. Also if you get a lower floor room, you might have a better shot at getting a WiFi signal which we could not get from the 4th floor. I would not recommend our first kind of room which was a toilet in the shower room set up. Personally I'd go with the walk in shower with the jets (which did not work on our shower but that was not an issue.) Electricity is regulated by your key which you will stick in a box in your room and that turns on electricity outlets and AC which in the Hot Barcelona summer was much needed, but never really got fully going it seemed. The attendants and personnel were pleasant and helpful.
Overall you just have to know what you are getting into: a hostel, not a Hotel... if you are used to even an American Holiday Inn room, this is not one of those. It is small, but it's in a real old building, so understandable. Just go with an open mind and pack light because there's not much room for luggage. One thing it will do is encourage you to venture out into the city instead of staying in the rooms. Barcelona as a city is super great! This hostel is a good hostel.