They're meant to be firm to take the weight but soft to sit on...if that makes sense lol.
I used one before I went into labour whilst watching TV at home, it was great being able to wobble and bounce around.
I also used the hospital balls when I went in to be induced, it's great for the pain because like Yvonne says you can sway and move rather than sitting still through the pain, but I had a lot of back labour and it didn't help that
At one point it was 3.00am, I waddled down the corridor and carried my ball into the cool dark family room in the hospital and sat there bouncing and swaying alone as everyone else was in bed. Just the odd nurse walking past. Must have been a right sight lol.
Only trouble I found after sitting on them for more than 30mins when I got up I felt really heavy down below, which I suppose is the sudden weight hits you again when you get up
Cx