I'm an ASU Tempe student and just barely heard about Tapacubo during a search for something new in Tempe. Went with a friend for lunch during the week not too long ago and was thoroughly impressed. There wasn't a single other customer when we went in, but others joined us every once in a while during our meal. The personnel were friendly and seemed knowledgeable.
As others have noted, the menu is fairly short, however the selections were sufficient. The prices were average, probably a good deal for the quality of the food. My friend and I each got two tacos, different flavors. I enjoyed the chicken taco, but was most impressed by the "El Papi" (new favorite). You DO have to enter the hotel to enter the restaurant.
The place had a sort of Route 66 gas station vibe going on that almost seemed like a modern street taco stop in LA. Loved the food and the atmosphere. I will definitely be coming back here as soon as possible.