"All Margarines are not equal-especially when it comes to baking cookies. For the best results, choose stick margarines containing at least 60% vegetable oil. Spreads with less than 60% vegetable oil have a high water content and do not give satisfactory results in any type of cookie, including bar and drop cookies. Also, avoid spreads labeled diet, whipped, liquid, or soft; they are intended for table use, not for baking. Stick margarines containing corn oil will produce a soft cookie dough, so you may need to adjust the chilling instructions given in the recipe. Do not substitute cooking oil for the margarine in cookie recipes. Oil is 100% fat; margarine is an emulsion of fat and water. Substituting an equal amount of oil for margarine will produce a cookie that tastes and feels greasy."
Well there you go. I didn't know all of this!