Los Gemelos is AMAZING. I live only a block away and have been going there for about a year now. The fresh salsa are very, very, good but beware the spiciness if you're not used to.
If you are hungry for some really fast, cheap Mex fix, then go for a couple of steak Sopes. If you are the kind of person who wants rice and beans and make it up yourself, you want the fajitas.
Its seriously good food. I looked in when it first opened and was like "Wow, this place looks really cheap and shitty" but when I went there the first time (only because it wasn't the only place packed for the women's soccer world cup match) I decided to order a taco and was very, very surprised!
Moral of the story? Don't judge the place by how it looks. It's like a dive bar equivalent that makes really solid drinks. You won't be disappointed.