Dunno how recent this thread is, but as I have mentioned a few times, my entire '72 Chally is fibreglass. fenders, doors, quarters, lift-off deck and hood. The only problems I have found so far is that A) the doors are sagging. I will need to have them realigned properly, but this should be an easy fix. and B) the lift-off hood does not sit flush with the fenders so I have to 'build up' the inner fenders to accomodate this. Also, the hood bows SEVERELY at high speeds, and once even popped the hood pin out of the fibreglass inner fender. I have since rigged up a sub-frame for the hood, but this is still an issue.
I recommend that if you go all glass, make sure the hood has hinges and latches to the cowl to avoid this severe bowing. Other than that, I think the glass makes quite a weight difference.