Can I always use butter instead of margarine when baking?
Margarine has less fat than butter, but it doesn't give quite the same flavor as butter does. You also have to be careful what KIND of margarine you are using.  Tub margarine has a higher water content and can ruin your baked goods and the stick margarine can have a lot of trans fats in it.  If you really want to get detailed into the differences, check out this site:

http://www.baking911.com/pantry/fats.htm

I agree with everything stated on that site in the fact that butter, in my opinion is far superior in flavor and texture than margarine.  I have yet to meet a dish that butter didn't improve the taste over margarine.  But there is the thing, it really is a lot about taste.  A good quote from the site about margarine:

"Not great for baking: does not allow foods to become flaky, rather more cakelike; does not spread as well as lard or shortening; adds a greasy taste. However, some bake with it all the time and have great success; it's a matter of taste."