Depends really. They add extra one sided weight (depending on which side you mount it on) and can really only perform to it's maximum potential with a fast loader like a halo or eggy. So it begs the question why bother if you can buy a halo and eggy that will load faster than a warp feed.
They do look the dogs bollocks and can reduce hopper shots and give you a better view over the top of your gun. I would say try before you buy if possible.