I have to admit I'm not happy they went back to aluminum. I know, everybody thinks I'm nuts.
My powerbook is aluminum and it got bumped the first week I had it and was damaged. It was also in a very cushioned case when this happened. It's not like I dropped it or anything. To this day, it's damaged even though we tried to fix it and did as best we could.
Aluminum is just too soft and pliable. With the plastic, I felt like it could handle a bit more. Now, when I get my new macbook next summer, I'm gonna constantly worry again.
Again, I know I'm going to get flamed for this, but it is my personal opinion....so try not to be too tough on me.