I'm in love! I can't believe I didn't get one of these 10 years ago!!! First, I watched some of the ball exercise videos on YouTube, just to get a feel for what you could do with the ball as I got into better shape. In one video, the woman showed warmups sitting on the ball, and her thighs were at a 90 degree angle to the ground. I took out my tape measure and measured how long 65cm, 55cm, 75cm were compared to my pants inseam to get an idea of how big the ball is inflated. Keeping in mind that the ball will not be fully inflated (60cm), and will squish down when sat on or used, I figured that the 65cm ball would be the ideal for my 5'6" frame with a 75cm inseam. PERFECT size! When sitting on it fully inflated, my legs are at a perfect right angle to the ground.I am using it instead of my desk chair. The first day was awful. Like most people, I slouch in my chair, and despite periodically reminding myself about posture, if I'm busy doing something, I end up slouched like a bag til I jerk myself up and remind myself of my posture again. Sitting on the ball makes this reminder easy. I am NOT in great physical shape, so balancing on a ball all day long at my desk was painful and hard to do. My muscles ached, and I couldn't find my bed fast enough.But the 2nd day, it was easier, and now, the 3rd day, I'm almost used to it. Just like that, I can see and feel my core tightening and there were several surprising side effects. I got up from my desk a lot to move around. Sitting isn't 'easy', so standing and walking was just as viable an alternative. This forced me to move nonstop all day, yet never stressed me out. It is proving to be a gentle building of my very out of shape endurance. Other side affect was the effect on my vision. I was used to looking at my monitors all day from exactly the same height. So when I stood up, I sometimes would get dizzy from the change and adjustment. Well sitting on the ball, I sometimes bounce and 'march', which makes my vision jar up and down. This was a weird effect the first day, and made me a little dizzy, but now, 3 days later, my actual balance has improved overall too. Did I mention I LOVE this ball?I let my son, who's way more physically fit than I am, try it, and he gave up after 10 minutes, so maybe it's not for everyone. He said watching videos while bouncing around a bit messed with his eyes. That's exactly the effect I had, which is why I kept doing it. My brain adjusted and that's exactly why my balance improved. My first day was TORTURE, but I was determined to get more exercise while sitting at my desk turning into a vegetable.I figure if I use this as a desk chair for a couple of weeks, I will build up enough core and strengthen my muscles enough to try the video exercises. Not there yet, but I can see it happening. The ball itself seems very durable, but if I had to buy a new one every week, I'd consider it well worth doing, because it's a whole lot cheaper than a gym. But it looks like it's going to be fine for a long time. It was hard to pump up the first time (pump included in box), and really hard to get the pin back out when you want to top it off the 2nd time (but they included 3 cap pins in the box).I bought this ball, having never seen one in person before. I think everyone should get one, these things might single handedly cure overweight America if everyone were forced to use them as chairs!!Helpful my comment so I know people actually read it please!