Roberto's is my favorite taco joint in Las Vegas. This is the location we typically go to, but there are locations all over the valley. Some locations are nicer than others, but the food is consistently tasty.
I normally get the carne asada plate and split it with my boyfriend. There's usually enough for the both of us to make 4 or 5 tacos from it. The steak is always flavorful and juicy and it also comes with beans, rice, and lettuce. The flour tortilla that they give has a great taste to it - I could eat it alone. We always get the chicken tacos too. They make the taco shell fresh so its crispy once you get it. They pile on the cheese too (my favorite part) ;)
I go here often and whole heartedly recommend it if you in the mood for some fast authentic Mexican food.