I purchased this 65mm TKO ball after seeing Spri balls promoted on tv as a good alternative for an office chair. I wasn't sure how much I'd like it, but wanted to make working at my desk more physical and for $15 it wasn't much of an investment - far cheaper than my leather chair. I've been sitting on the ball between four and six hours per day for two weeks now and it's holding up well. There hasn't been any leakage in that time and the ball is very stable to sit on. Even on a glossy tile floor, there's no fear of it rolling out from under me.
Sitting at a desk for long hours, my tailbone often gets sore, even with those u-shaped pillows, which isn't the case when sitting on the ball, nor is lower back pain an issue. I've found that sitting on the ball seems to promote proper posture or rather it discourages anything but. You can't sit on it lazily. Crouching will lead to upper back discomfort and possible neck soreness rather quickly. But, I guess that's why it's better for you, to make you use more muscles and make sitting more active. It might be best to ease into it, rather than going nearly cold turkey like I've been.
Regarding the complaints that a few others have had with the ball being smaller than advertised or lopsided, I believe that's a result of a failure to fully inflate the ball. The ball is made of very tough, but flexible material that I imagine would be a pain to fully inflate with the small hand-pump that's included. Even with an electric pump that we bought to inflate our portable matress, it took a good ten to 15 minutes to inflate to the 25" diameter recommended by the manufacture. The ball may appear fully inflated at only 21" diameter, but there will still be a few creases giving away that it's not. And, because you're not just filling the cavity with air, but also having to expand the strong rubber shell, that last 4" seemed to take as much time to inflate as the first 21", though I spent more time at the end measuring to ensure the ball didn't over-inflate, so the time could be deceptive. Once fully inflated, the ball doesn't appear remotely unbalanced or asymmetrical, nor was mine smaller than advertised.
If I can ever get conditioned to sitting up straight for long periods, this should prove a great investment. Regardless, for a ball this large and durable, $15 is a great value. compared to many others.
After months of use, I haven't needed to add more air, even once. I've used this ball as a chair, as an excercise component, and my neice and nephew have enjoyed putting it through a more vigorous testing of bouncing on it more abusively and it's still holding air extremely well. The upper back soreness I previously mentioned had less to do with posture and more to due with a 9000 mile motorcycle trip I took the year before. I've had minor upper-back problems, generated from my years in gymnastics and many more carrying heavy loads in construction. It seems it was past time for an alignment.