I have the "blue with red flames" one on my 12" alubook. I dig it. It will help against scratches, makes your powerbook look either stupid or cool (depending on opinion), and won't permanantly alter your hardware. The glue that attaches it is the same stuff as that rubber-cement like stuff that you find attaching flyers to magazines (you know...it allways seems sticky and is really stretchy), so you can remove the skin and your powerbook is back to normal.
The only thing I can see negatively about it is that on the corners of my powerbook there seems to be some scraping off of the paint in the skin. This is not on the top, but on the overleaf, so it isn't really noticeable.
Here in Japan people love it.
What is the best one? That's up to your tastes, isn't it?