Great tutorial! I used the mirror effect to create a Mantis Valentine. Bugs have an exoskeleton so I fudged the assignment a bit to make it instead of the assigned skull. I had a little problem because I drew a sketch of what I wanted and used a photo of that drawing in Illustrator. I had it at the bottom of Layer 3 and locked, but it would still get selected if clicked on. Very bizarre and I couldn't figure it out. I took it out of the group eventually so I could stop selecting it.
Also I had no idea you can change which is on top, stroke or fill, that kinda blew my mind! I always just end up making more shapes and using pathfinder.