I really think this depends on your upper body strength and ability to tolerate the heavy weight on your shoulders. Right now, I use 70-80#, depending on my strength that day. I just did Leaner Legs and tried lifting 75# and knew it wasn't going to happen, so I dropped to 70 and had no problem lifting it. It took a while to build up to this poundage, though. I felt like I was stuck at 70# max for a very long time.
Erica