If I recall, Nancy's stuffed was good way back when. They have, however, expanded to be more of a Domino's-like chain, with little stores in strip malls, and the pizza has suffered tremendously (IMO). I tried the thin crust about a year ago (at a Nancy's that has subsequently closed), and it was awful--below average. I assume the deep dish (if they even have it anymore) is the same.
I won't swear to this, but Eveyln Slomon's Pizza Book might have a Nancy's recipe. Be warned, though--her recipes do not produce restaurant-style Chicago deep dish, because they assume overlong kneading times.