As others reviewers noted, the chart that comes with this is pretty useless. But, that's not why I bought it. I wanted a large, cheap exercise ball, and that's exactly what I got. I do find it rather strange that the pump comes with several different tips, only one of which actually works with the ball. Ah, well, more little pieces to have lying around.

I LOVE MY SPRI BALL. ITS REALLY WORKS AND ITS ALSO A STRESS FREE TOOL!

The ball quality seems to be pretty good. I am 5'5" and it is a bit tall for me (I got it for my boys and for the pump). However the 55 cm is barely tall enough, and I may actually use this one for some of my own things. The 55 only goes up to 21" when inflated, per their instruction sheet. I may let this one deflate to 22" and use it. It isn't quite as thick as the Elite, but seems to be a good thickness, and suffice. I tried some exercises on it and give it a thumbs up.

The pump is the reason I gave it a 4 star instead of 5. It took a long time (minimum of 20 minutes) to pump. I did it while watching t.v., though, and it wasn't a big deal. It does fit the ball opening well. I used my pump from the everlast base and stretch band kit, and it was MUCH faster! It only took 5-7 minutes, but the tip did not fit into the ball opening as well, and was a little tricky.

I was looking for an exercise ball to add to my trampoline workout. SPRI was a good brand that I have purchased several other items from to get professional quality products. I was very disappointed with how flimsy this ball is. It is a soft, lightweight gauge that doesn't feel sturdy enough to support any weight and it squishes down too much.

Exercise books say that the correct size ball is determined by when one sits on the ball, one's thighs and shins should form at least a 90 degree angle: thighs should be more or less parallel to the floor. DVDs I've seen show demonstrators using balls where the thigh-shin angle is at least 90 degrees or greater.

People who are 5'4" or shorter should use the 55 cm ball. I'm 5'3" but found the 55 cm ball to be too small. Even when inflated to be quite taut, my hips are still lower than my knees when sitting on the ball. Perhaps a consideration is one's weight relative to one's height? If one were relatively lighter, one would not compress the ball quite as much.

With a bigger ball, one could inflate it less to achieve the correct thigh-shin angle whereas with a small ball, the only recourse is to lose weight.

On the other hand, when lying on the ball to do chest presses, a smaller ball would allow the elbows to reach closer to the floor thereby expanding the chest more than with a larger ball.

A smaller ball is more challenging -- there is less distance to travel before one would fall off, so it is harder work to use a smaller ball.

The pump is a joke, a real bicycle pump would be more efficient. The ball's valve is also a joke -- not well made but it's good enough, it works. The chart is useless -- spend the money on an actual exercise book.