The fish tacos are small but tasty. I don't really think of this as mexican food, but it's definitely clean, streamlined, and Americanized. Not my first choice for mexican fast food, but I'm more the La Tolteca / La Salcitas type.
Don't let me turn you off-it really is pretty a-ok.
If you are scared by grubby looking dining places, then Rubios is a better choice for you. It is a clean, attractive, chain restaurant. I'm pretty sure that it's Number One for decent fast food American/Mexican style.