It has authentic Mexican decor and cuisine (not pseudo Mexican, where it looks Mexican but tastes Indian). The smoked beef burrito especial was excellent, as was the margarita. If you're looking for local Bahraini flavor, this would not be your restaurant of choice. It’s a big expat destination - I've never seen so many Americans at one time since I've been here. I could easily have been in Texas, except the staff was all Indian instead of Mexican.
The restaurant faces Adilya Avenue, but you have to access it from the side road in back. Senor Pacos is on the same side of the street as the Berlitz office. They have a nice patio for when the weather is cooler.
You really only need a reservation for Thursday and Friday nights.