Cotton is very cheap and it works well. The Weber stuff does have a few things going for it. First, it seems to be utterly non-hygroscopic; you can soak it in water indefinitely with no ill effects. I don't think you can get enough PJ into cotton to duplicate this trick, and if you could it would probably very difficult to light. Weber also burns very hot; it seems to me hotter than a PJ cotton ball but I've not actually measured the temps (no IR thermometer). Weber cubes are also easier to light (depending on how you make your PJ cotton).

Another cool thing is that you can scrape off just a little if you don't need a whole cube. You can also put out a block that's burning and reuse it later. It's instantly cool to the touch, just like WetFire.