I actually dig the patinated look of my dented and scratched old MBP and my current MBA. My plastic Dells from work always look terrible after a few trips living in my backpack and getting banged around. Haven't had actual usability issues with either though.

I have.

Plastic is great, until your laptop falls and the plastic shell shatters. That's the weakness of plastic - it's brittle. I have a ten year old macbook with a dinged aluminium chassis. The structure of the shell is still intact despite a few falls.