I had a birthing ball with both mine, i loved it i used it to labour with as well.
In pregnancy it helps you to keep your pelvis open to encourage the baby to move down, (its the way you sit on the with your legs open!)
During labour they were fantastic to roll about on during a contraction, i was leant on the bed sat upright, it helps baby move down but also i got a horrible heavy pressure feeling towards the end and sitting on the ball eased that as it didn't feel like my bits were going to fall off!!
I would recommend one