My favorite detail about El Sombrero is the salsa, which tastes fresh and homemade and keeps coming with hot chips as long as you'd like.
My experience with the rest of the food (vegetarian options only) has been somewhat hit or miss... the vegetarian burrito is pretty good, and I've ordered it smothered in their unusual and good creamy chipotle sauce (usually reserved for a chicken dish), but the avocado tostada was a disappointing mound of iceberg lettuce and avocado slices. I don't know what I was expecting... maybe that's what a tostada is. Also slightly disappointing is the fact that they have homemade tamales, but none that are meatless.
I've heard decent review of the meat dishes, but have to say that the vegetarian options are a little bit lackluster compared to the options at other places around town.
The staff is pleasant and consistent, and seems to be headed by a matronly woman who always looks beautiful and in charge.
There is also a full bar that makes blended drinks, which is a nice addition, and they don't use any lard in their re-fried beans.
Edit: El Sombrero has awesome vegetarian spinach enchiladas.