I have heard some bad on fiberglass springs.they can't take heat from exhaust and stuff..they can get brittle ,they can crack from rocks and stuff hitting them.
But they are also lighter..notice I think hyperco makes some composite ones..anyone running them.Also do they still work with normal traction devices like the slide a link or caltracs..

 

I ran fiberglass mono-leafs on my 68 RS and never had any problems. I think someone is guessing on the heat and cracking issue, because I certainly never saw any of that.

 

If they can perform well enough in your application I dont see why not run them. Some cars have used them for years with no problems. Also, after recently snapping a steel leaf in half I am less confident that they are nearly invincible .
CoryM