definatly now. not only are they FREAKIN EXPENSIVE, but they restrict gas exchange in your tank, and if you have none or too little, if could kiss everything in there. if you have a reef it also cuts down on PAR.
if you have fish taht jump out, get eggcrate. if you have a reef and florecent lighting (pc, vho, t5) but NOT MH, get eggcrate where the parts are triangles, point facing down, this will increase the amount of light that enters the tank that can be used for corals.
i vote a big hell no.