Cream of anything soup will work. I often substitute c.o.mushroom with good results.
The cheese does not get stringy, but rather blends in with the creaminess of the soup & sour cream, and the potatoes soak it all up. The dark brown, crunchy outer layer around the pan is the best part.
Occasionally I will add chopped parsley, diced ham or sliced mushrooms to it. It lends itself easily to many variations. It's hard to screw up.
I have also made the casserole without first thawing the potatoes...it took a little longer to cook, but end results did not differ.