Having lived in Texas and spent some time in San Antonio, I think San Antonio's Modern Mexican represents the famous Riverwalk well: good drink specials and average food in a dimly lit environment.
It's a good sized place with lots of seating. However, once the place is filled, it is difficult to hear your party since it gets quite loud.
We ordered the chicken fajitas. What convinced us to stray from our veggie habit was the description of being marinated for 24 hours. For something that has been marinating that long, you would expect it to pack a flavor punch. The fajitas were nothing special, nor had any exceptional flavor. The salsa seemed bland. The chips and tortillas were good. Again, wanting to represent Riverwalk food, they got it right.
The service was good and friendly. The main draw is their drink specials...$1 margaritas every day and at any time!