I personally use a rucksack, but Bubba's right it does interfere, and I have to rest the sack on something in between sets because it does hurt like f*ck. But I like it better than having weight swinging around by my feet, so I guess it's a preference thing.
Mark Croxall uses an old life jacket filled with sand, and it obvioulsy works for him.
Maybe there's a profit to be made in this.