Polyfill is just plain pillow stuffing. If you put it in your box it can make the box seem bigger than it is to the sub. I think it works good. And you can get it just about anywhere like walmart it will cost you a couple bucks. Only use 1 lb. of polyfill per cuft of airspace.
What opinions, what Isaac said is what it is. Polyfil slows the travel os air/soundwaves within an enclosure tricking the sub to thiking it is in a larger enclosure. It also breaks up standing waves which helps to internally dampen the enclosure. Polyfil is mostly used in sealed enclosures but has also been used in ported which is not recommended. The use of polyfil in a ported enclosure may impeed the ports performance so if you need dampening use thin fiberglass matting stapled to the walls of inside of the enclosure if you need a bigger enclsoure, build one. In sealed enclsoures it works nicely and can help a cheap box sound good. Polo..
also, try flipping the sub upside down... it'll eliminate the displacement of the sub thereby creating more usable volume in your box. I just point the box the same direction, but try as many as you can.
Some people say it gets sloppier sounding that way... i don't know if i agree or not... there's definately a difference, but PERSONALLY i think it's a good one... not to mention that the addition of polyfil with an inverted sub really makes the whole thing bump a tad more than you might expect.
try it... on a sealed enclosure. it's got other "pros" too, but thats another thread.