I've only recently got myself a lw body shell and have not even used it yet. I can however immediately notice the massive benefit it will give you over a standard weight.
With the new shell sprayed up, it is coming in 22g lighter than the normal weight. Although this doesn't sound much, if you think about it, this is unsprung weight, plus you have got to take into consideration g-force.
These cars are probably generating in excess of 5g when going round a sweeper at the end of a straight, if you say conservatively they generate 3G, this then makes the actual weight of my new lw shell 66g lighter going round a bend! When you then think about the position of the unsprung weight.......that is a huge amount!
In terms of durability, they do seem very "soft" in comparison, even reaming out holes, the reamer seems to push the lexan out of the way rather than file it, so I think they will bend and dent a lot easier but won't crack as easy, and it's the cracking which really reduces the life!