Done right, it's an amazing, flavorful filling with a relatively thin shell of special ground and leavened corn flour around the outside. Most of the bad ones i've had were 90% rubbery unseasoned masa with a thin stripe of meat down the center (cheap). I've been practicing for ages and i'm only up to "not bad" on a scale of 1 to 10 :)

Mexican food at a lot of chain places is made as cheap as possible, and while cheap tacos are still fucking delicious, it doesn't translate well into every dish.