I like Rosa's.
Being that I've resigned myself to never again eating the great cheap Mexican food I had in Chicago (or even in Madison), I might have just lowered my standards -- but I think that Rosa's serves inexpensive and tasty food.
I know some other reviewers have complained about the atmosphere, but I think the place is cute. Also, our waiters have always been really friendly. The last time I was there, we were served by a guy named Juan and he was the most outgoing, responsive and sweet waiter I've had in a really, really long time.
I've only been here for lunch, so I usually have the lunch specials, which are in the $6 range. I feel like I get a pretty good deal, and the service is quick.