Checked out this place last night with a group of 8 after reading all the raves here. We brought our own beer which was fine with the staff.
I have to say after reading about all the great Mexican specialties served at the opening party, I was rather disappointed with their regualr menu; no special items in sight. I even asked if they had any that were not on the menu. The only special that night was a veggie burrito.
Meat choices are only carne asada and chicken; no carnitas or al pastor or anything else. They do have fish tacos, but three members of our group had them and we all agreed that the fish was bad. Not edible.
Chips were very good (light and crispy) salsas were good, guacamole was very good, as well as the black beans (well seasoned).
The corn tortillas are homemade and tasty. The tacos however were too large for my taste, and overloaded with pinto beans. The steak was very good, not the well done steak you get at the taco trucks, it was served medium and was higher quality, like you'd get at Dona Tomas or Tacubaya. And it's reflected in the price; asada tacos are $3.50 each.
The fish taco is $3.95.
The fajitas got the most favorable reaction from those in our group. The chicken quesadilla was tasty, but a bit lacking in quantity of chicken.
I think Las Mesas is good, but I hope they put a little more diversity into the menu in the future (carnitas, al pastor, rellenos, mole, etc). I know the kitchen is capable of great things - it's just that the menu seems very "safe" right now.