I found this on the Better Homes and Gardens website. I substitute all the time, and do find that the consistency sometimes changes, but not enough to ruin what I am baking.
"Generally, you can use butter or margarine in place of shortening, but making this substitution may slightly alter the texture of baked goods.
Shortening is 100 percent fat, but butter and margarine are composed of about 85 percent fat and 15 percent water. Though this additional liquid may change the consistency of the sweets you bake, butter and margarine’s rich, pleasing flavors and texture usually outweigh the disadvantages.
Cookies made with butter or margarine may be softer and spread out a little more. In cakes and breads, the substitution is rarely noticeable. Piecrust made with butter or margarine not be as flaky as one made with shortening."
Do not use whipped butters, margarines, or blends in baking.