I'll be the first to admit that outside of Burrito Boyz, I've never really had an "authentic" burrito.
I will say that I thought the Burrito Mojado I had was quite good. The beef chunks were good and the rice was delicious. The side salad is a bit lame but I never expect much out of side salads. I also heard good things about the chorizo taco (better than beef) and side rice.
I think the biggest vote of confidence I can give is that I will be back. I want to try and tacos, nachos, and the fajitas. I think if you're looking for authentic (at least I think it's authentic) Mexican food, you can't go wrong with El Rinco.