I just filled a backpack with about 75-80 lbs. and have been doing squats and shoulding (floor to should lifting) with it and I'll tell ya, thats the heaviest 70 lbs. I ever moved. For wrestling I'll use a backpack with 15-20 lbs. or hold a couple 35's and practice my shots. Good stuff. Very hard.
Member of DaJoGen MMA school under Dave Hagen and Team Chaos fight team under Denver Mangiyatan and Chris Toquero, ran out of Zanshin Martial Arts in Salem Oregon: http://www.zanshinarts.org/Home.aspx