Burritos Express is a great new addition to Downtown Crossing. The food is very inexpensive. I buy the cheese and bean burrito, and spend less than $5. The burritos are huge and VERY TASTY. Staff is very friendly. Orders come out very quickly, and you can call ahead and pick it up. I also like it, becaus it is never very crowded. I think this is because it's new and a little out of the way, so not many people know about it. I think their burritos have much more flavor and are more authentic than Qdoba on Summer Street. Also, the lines are shorter and it is less expensive.
I have not tried the quesadillas (as I am hooked on the burritos), but everyone in my office eats them all the time.