I bought the medium sized one while pregnant to use as a labor ball, but the "labor" part never happened! lol (Ended up with a c/section). However, I used it like crazy during the last part of my pregnancy and I loved it. It's great on the back! I had horrible backpain and it really helped. I found that sitting on it while it was very easy, no problem at all.
I've just now included it in my exercising and I like it for that too.
I believe the medium one holds 350 lbs.