First thought: Awesome, Vienna is really lacking in Mexican fast food. Unfortuately, then I tried it...
Prices are cheap, but at least so far the employees seem to have no idea what they're doing. It's pretty much built to resemble Taco Bell, but the quesadilla I had (which was heated -- or rather mildly warmed -- in what appeared to be toaster grill) did not even come anywhere near to the quality of the US fastfood chain: In fact, it was so bad I did not finish it.
I might give this place another try in a few months, but in the meantime I'll stick to "Fresco Grill" in 1090.

The Quesedilla I had (which the menu claims isn't availible anywhere else outside of Mexico) was good, they're open until 2 am, and supposedly they also have wifi.
You might not want to sit right at the windows, since a Trafik directly across the street has put up a distracting LCD that continually shows commercials. How lame.