I've been to this restaurant twice since it opened. I'll have to hedge on an unqualified vote of confidence in the food at Tortilla, but they seem to have the right idea when it comes to customer service. I grew up in Southern California, by the way.
The ingredients are fresh, and every burrito is prepared fresh in front of you, using chafing dishes and containers much like you might find at Subway.
You get a choice of two kinds of rice ("Spanish"--actually Mexican, and lemon/coriander) and two kinds of beans (pinto or black--go with the pintos for a truly Californian experience).
You should also be able to choose from three kinds of meat, but I haven't yet had the carnitas (pork); it was not available because they were still "perfecting" the dishes. Carnitas is by far my favourite burrito meat, so that was a disappointment. The other options are, of course, chicken and beef (carne asada [de res]).
The hot sauces aren't hot and you shouldn't take their advice for the pairings. The salsa verde is better with pork than with anything else, but I'll always go for the hot sauce.
There are other ingredients as well. Guacamole costs 50p extra.
They have a tendency to overfill the burritos so that they burst when you are eating them. This is a sign that the staff are still learning the ropes. Try asking for less rice, maybe, since it's not really an integral part of a burrito.
The staff also got the order wrong on three of the five burritos we have ordered here. The mistakes were fixed, no questions asked.
Their rubbish disposal system is abysmal, with frustrated customers just piling the rubbish up on the bins.
I recommend this place, but I think the burritos at the place up the road are a bit better (and the hot sauce is actually hot!) It's a pretty close call, though, since I think that Mucho Más overcharges on guacamole (£1).