[David Brinza]

You can get tremendous stiffness-to-weight benefit from carbon-fiber composites, but there are drawbacks. As already pointed out, composite structures are susceptible to impact damage. Assembly with composites can be a challenge. It's rather difficult to use fasteners with composites. If you want to use screws, you will need to install inserts or have large nutplates bonded to the composite material. In order to use rivets, you will need to bond sheet metal to the both sides of the composite around the area of the rivet (directly riveting composite will just crush the material and result in a very weak joint). The preferred method of joining composites is bonded joints and to do that right, you need to control the bondline thickness.

That being said, there are some structures that can be effectively constructed from composites. High-stiffness tubular truss structures are a good candidate for composites. Holy Cow's (1538) forklift arms in San Diego were a work of art, Unfortunately, I didn't get a photo but maybe the San Diego Imagery Award winners can share this.

From cost, strength-to-weight, ease of fabrication, assembly and repair perspectives, it's hard to beat aluminum for FIRST robotic applications.