I've been to Guajillo at least 5 times in the last 3 years, and it's been hit or miss. Having grown up in Southern California and lived in Texas, I would say calling it Tex-Mex is a definite misnomer. You do have to give them credit for having some real Mexican dishes, not the cheese laden, over-sauced fare which is called Tex-Mex.
On the other hand, I would have a hard time saying most of their food really has the flavor of "Authentic." On one occasion, the ceviche came in way too spicy, like eating salsa with habanero peppers. And as others have observed, the chicken has been often a bit dry. I also find most of the time the food is slightly underseasoned.
Having said this, I would not rush back to Guajillo. In my experience, I have always wanted it to be good, as the menu reads nicely and offers up some very authentic sounding dishes. But the flavors just do not shine. Enough said for now