Finally got to visit Cancun Express in Downtown Crossing, which is now called Burrito Express (don't know if there was an ownership change or not, but the Chowhound recommendation that sent me here isn't that old).
While it was a little slow for a lunchtime place post-rush, it was fully worth the wait. I had the Tacos Al Pastor, which typically are shwarma-like vertically rottiseried and sliced marinated pork or lamb, here I think were maybe just cubes of marinated pork, but completely delicious. Homemade, grilled flour tortillas, chunks of pineapple (!), fresh cilantro, shredded lettuce, shredded white farmer-ish cheese, and a rich and slightly fiery orange chili sauce on the side.
Three of these, generously filled, plus black beans and rice, and a little homemade-tasting salsa, for $7. I shouldn't have eaten this much in the middle of the day, but it was so good I stuffed myself. A real standout in a sea of dull, indistinguishable burritos and wraps. I had to tell the staff how amazing I thought the tacos were. I'll be coming back here very soon.