I visited Barcelona last summer and spent 4 nights in Mino Hotel. It is a very nice hotel, with very good prices. I have been in Castelldefels more than once, and in other hotels. Services were great (TV, terrace, air conditioned, rooms equipped with Internet access,...). Rooms are also very clean. It is next to a highway, so you can go to Barcelona in about 10 minutes. It is also next to Castelldefels beach and a train station, where trains also go to the centre of Barcelona.

It's not a luxury hotel, but I would recommend it for most people. I'm planning visiting Barcelona next Summer...so you just know where I'd be.