95 used a fiberglass backed cardboard honeycomb composite board this year, in places we would have otherwise used plywood. The main use was as the electronics board, which ended up becoming another structural member, helping to tie the frame together.
We mainly used it because we found an appropriate sized piece that a sponser had left over. It was attractive not only because the price was right, but because it is extremely stiff in most planes and weighs a fraction of the equivalent piece of plywood. It is the perfect material for mounting internal components or as a structure that needs to be rigid in the appropriate dimension. It is also non conductive, easy to work with and for all purposes transparent to radio waves.
It also lends a certian 'high tech' look to the 'bot. I like that. Carbon fiber is cool, but I would fear my robot would end up looking like a K&M cold air intake
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There are lots of ways to use non metal/wood/PVC materials. Composites are great. But I still like a good aluminum or bracketed plywood frame any day. It's tried and true technology, and it can be cheaper in a lot of cases.