this ball is not good if you aim to sit on it, might be fine for pilates or gym stuff but for me it was to be my office chair as I have a bad back. The ball doesn't hold its shape when you sit on it, other balls maintain their spherical shape this one goes like a sideways egg which means that 75cm suddenly becomes closer to 65. I had it for less than a wee when it burst, and at least the anti burst worked as it did go slowly down so that's something! I will not buy another one. It is also horrible material which picks up dust and is not easy to reposition yourself on if you are in normal clothes.
I bought this item for my wife because she found it hard to get comfortable sitting when she got '6 months plus' pregnant. It's been great and helped her no end. In fact it's become the seat for the computer table too now, it's that comfortable.
The negatives are, you'll need a good pump to inflate it (I used a stirrup pump that I use for my bike) and the needle valve that comes with it is a bit rubbish (though it works), so I'd consider getting a decent one of those too just to make your life a little easier.