Generally speaking, butter spreads have a water component to them. That's what the emulsifier is for, as it keeps the fat and water from separating.
The problem is that you can't fry/cook with it. If you place it in a pan it will separate, and sizzle in a very unpleasant manner. If I remember correctly, it actaully smells quite awful in the process.
As for baking, I think it'll be fine anywhere there needs to be fat as fat. Wherever you need fat as something to hold the structure, you had probably best not use it. Caveat emptor, as I don't bake very much at all.
Let it be aid, however, that margarine as opposed to butter spread, can be used anywhere that butter is used. It isn't as tasty, and has trans-fats. On the other hand, it has less cholesterol.