some body panels on 2008 335 seem to be a composite. what are the benefits of these?

Less weight, corrosion resistant, more resilient.

and some cons...

*Can crack easier in minor collisions
*Usually has higher expansion/contraction than the other materials around it (although I haven't noticed changes in door gaps).
* Paint will look different on different surfaces (maybe this is why we have orange peeled paint?)

According to design info, composite front fenders on 335i coupes was done to allow 50/50 weight balance.

According to design info, the composite front fenders on 335i allow 50/50 weight balance.