You know a place is good when the employees in the surrounding businesses suggest it, unsolicited. I get my hair done next door to Rivas, and there's always at least one stylist eating nachos or a burrito and it's not unusual for the whole store to erupt in arguing over favorite dishes.
The consensus seems to be that the fajitas are the best choice but you can't really go wrong. Their chile relleno is one of the best I've had and their burritos are packed with protein. Ask what's in them though, because the shrimp just comes with pico and guac (and a TON of shrimp!), and I opted to add spinach to fill it out. The enchiladas get really mixed reviews though.
Rivas is a counter service place with very limited seating. Don't leave without hitting up the salsa bar- selection is limited but the red is mild and the green is good for more kick.