I've used approx. 3 mm Alupanel in an FRC robot for the structural belly pan. It was alright, but ultimately needed lateral ribs that the quasi-monocoque design didn't provide for.
Its stiffness was better than plastic of equivalent thickness, but quite substantially less than aluminum—and although the solid core made it easy to rivet down, it meant that the weight savings were quite modest.
It cuts decently well on a CNC router, which is what we used.
If I had to do it again, and I
really wanted to optimize the part for performance, I'd find a suitable sheet of fibreglass composite sandwich panel (carbon fibre is perhaps too expensive for FRC), or make one from raw material. (There's a fair bit of up-front expense to make these, but you can always make tooling and refine the process in the off-season, and those costs don't count against the limits.) If you make your own, you can lay it up with a lightweight honeycomb core in the centre, and rigid plywood or foam cores where you need to attach mechanical fasteners.
More ideas:
http://www.chiefdelphi.com/forums/sh...d.php?t=121911
http://www.chiefdelphi.com/forums/sh...ad.php?t=98779