Regular margarine can be substituted for butter in virtually everything. But whereas margarine is 80% fat (like butter), many soft margarines (such as I Can't Believe It's Not Butter) can contain 60% or less. Some reduced-fat margarines have water as the leading ingredient. So no, they will not behave just like butter or regular margarine - and especially not in baking projects.

Many professional chefs seek out butter with 82% or even just 81% butterfat, and claim they notice a difference in their pastries made with the slightly higher-fat butter. We have a little trouble noticing that difference. But imagine trying to bake a cream puff (where the only ingredients are flour, butter, milk, eggs, and a little salt), and one of those primary ingredients is missing one-quarter of its essential nature!

It is almost as big a change as if you had used only egg whites and substituted some other mystery liquid in place of the yolks. You would not expect to get the same results as you had in the past with tried and true ingredients.

The I Can't Believe It's Not Butter website has a lot of recipes for you to peruse, but very few baking recipes. With a staff of recipe testers, home economists, and a full-time chef, the company clearly puts lots of effort into creating new recipes - and probably modifying existing recipes - to make use of its product. But most traditional baking recipes are probably beyond the scope of their modifications.