My first pregnancy: no way, I refused to try it. It looked scary. What if it rolled and I fell? I was scared of it. Then at a class for the 2nd pregnancy, there was all this stuff to test and try out. One was a birthing ball, and I was like "oooooh, no thanks" but someone encouraged me to sit on it. I made my husband hold my hands while I eased onto it because I was scared. DUDE! It was the BEST thing EVER! SO SO SO comfortable! I liked it better than a chair, much more comfortable. And it didn't roll, it was weighted and, well, made for pregnant people to sit on without rolling. It felt really nice and we bought one for my house. I'd sit on it and do little squat movements to help my legs, and it was supposed to help your body but I don't remember how or why exactly. But, I liked it a lot and would definately recommend it. I didn't use it during labor because my bloodpressure was high and they were monitoring all kinds of stupid stuff, but I used it to prepare for a month or two before, (that and a water exercise class at the local fitness center). They both helped tremendously.
After the baby and you heal up properly, you can lay on it and do crunches like you would with an exercise ball (actually there's a lot of exercises you can do with them once you're all healed up and ready to work out). Then your toddler can hold on to it and roll it around to practice walking. My 5 yr old and 2 year old love playing with it still, to this day, so definately worth the money----it's had a lot of uses and a lot of life.
ONE VERY IMPORTANT NOTE: do not sit on it until you heal. (LOL) That may sound obvious, but it wasn't to me. The day I came home from the hospital, my mom, dad, brother, son, husband, everyone was just sitting around and there wasn't anything left to sit on in the room except the ball that just happened to still be in the room. Noone moved to get up (I don't think they thought I was going to sit) so I just walked over and sat on that. YOUCH! Jer had to run pull me up because I couldn't move. Don't sit on squishy things that help you spread the day you come home from having a baby. Not nice.