I went to Alameda rock with two non-vegetarian friends on Friday evening. We ordered a number of different tapas (houmus, felafel, fried peppers, seitan sandwiches/montaditos, salmoreho...) and we shared them among ourselves. With a total of 6 tapas we left the place completely full and absolutely satisfied. The people are friendly, the prices are low (on average 3 euro each tapa, and considering that with two of those you are fine, it's not expensive at all).
We were outside so we couldn't enjoy the music, but they have good musical tastes too!
Great place, and I am glad I now have a place for evening tapas in Sevilla (the Veganitessen offers tapas but only at lunchtime)!
UPDATE April 2015
I went to the new Alameda Rock, which is now at the ground floor of La Caja Habitada, a hostel in Calle Credito. The place is small, but there are a few tables. Tapas are nice (essentially similar to what they were offering before, but they are adding new stuff as time goes by), and they have a big selection of drinks.