Commercial outlines are no substitute for outlines you prepare yourself. When you outline, you put the course together in your mind. Linking everything together in your outline ensures that you understand each concept, and where that concept fits into the course. When you buy a commercially prepared outline you only get the final product which is not very valuable. The really worthwhile part of outlining is organizing the information. The final product is just a bonus for having done the work.
Nevertheless, commercial outlines can be very helpful. Like the table of contents of a casebook, commercial outlines organize the information of the course in a logical way. Like hornbooks , commercial outlines state black letter law, although outlines are less in depth than hornbooks. So when you need a quick and simple review a commercial outline is a good bet. Also, commercial outlines might help you get an idea of how to put your own outline together. Finally, commercial outlines weigh much less than hornbooks and they easily fit in your backpack.
Outlines can be Judas goats, luring you to a slaughter. First, you don't learn the course like you would have if you outlined yourself. Second, they give you a false sense of security. You haven't struggled with the material, so you don't really know if you understand it. Finally, commercial outlines are sometimes wrong or the teacher may disagree with the outline author's interpretation of a particular issue. Don't be fooled, the teacher is aware of commercially prepared outlines and their contents. He just might test you where the outline is wrong, deficient, or in opposition to his opinion.
In conclusion, commercial outlines are great when you need a quick, light review. Or when you need a little help to start your own outline. Don't use them as a crutch because you haven't outlined yourself. Sounds like I'm talking from experience?