We got the FotPro yellow ball with stability legs that is advertised as 26"/65cm for our son who is about 5' tall and 75-80 pounds, so that's what I'm reviewing.My son's 4th grade class uses these, and they come highly recommended by his teacher. When we first visited the classroom immediately before school started last year, we were a little skeptical. She told us that they improve posture and increase focus, particularly for kids who tend to fidget (which accurately describes my son). After talking with him about it over the next few months, we decided that maybe it would help him at home too, so we got this one for his birthday last month. It turns out that these are the same brand they use in the classroom.The instructions for blowing it up the first time aren't as simple as "fill it up with air" and they should obviously be followed carefully.We also asked my son what the rules were in the classroom, and he said there are only two:1. Keep your feet on the floor.2. If you can hear your bouncing, you're bouncing too hard.These balls aren't toys, and those two simple rules seem to cover everything.We think it's been awesome for use at home. Our son seems to be able to better focus on school work when he can "bounce" -this way he's not tapping, picking, or otherwise doing things that aren't conducive to school work. (And every little bit helps!)For the record, I'm 5'2" and about 105 pounds. I considered getting one of these for myself to use at the office, but I find that it's a little too low for me. This might be because it's time to add air or just because I'm so short. (Remember that children's desks are lower than a standard 30" desk or table.)Well worth what we paid for it.I really think this ball is a 55cm as it seems really small compared to other 65cm balls I have used. Also the legs are a great idea but they just fold outwards when I sit on the ball. Overall, I wish I would have went with an inexpensive ball instead of paying more for a ball with legs.When I sit on it I do feel relief in my lower back/hips so it will do. However, my knees point up so I am not sure how long it will be comfortable. If I stretch out my legs it helps but the ball wants to roll.I bought several of these thru Amazon and a donor site to give to my friend to use in her classes with kids that have learning disabilities and who have trouble staying in their traditional seats. My friend reported that the smiles were immeasurable and for the first time ever the kids came together as a group. I personally have never sat on one of these nor do I know or understand its benefits for the back or spine. What I do know is IT WORKED for its intended purpose. The kids can focus better now and as mentioned have united for the first time. For that reason I give them a five star.I received these for my Kindergarten students. They have mostly been wonderful, just had one break (but to be fair, that was when a substitute teacher was in my class...so who knows the REAL story). The kids are able to move while they work. Great for 5-6 year-olds!Took a while to get use to it but can't sit imagine sitting in a regular office chair at my deck in the home office. Air has held up pretty good. I refill it every so often when it starts feeling a little soft.I bought this for my 6 year old boy - who is about the height of a 7 year old - to use while doing school work at his desk. His desk is average height (28 inches) but when he sits on this ball, he can't reach his desktop. I'd say that when sitting on this ball, his nose lines up with the desk.He still likes to sit on it while reading or just needing to get some energy out, so I'm keeping it. The legs do not provide stability - just buy a normal exercise ball - and get a stand if you don't want it rolling around.Like everyone said the legs fold up underneath it. I bought it for my grade 5 and 6 students ... yellow balls were perfect size.Had to replace for a class the only draw back is the description said 4 legs and this had 5 , which was a pleasant surprise since we were looking for 5 legs. Just wish they had made life easier and had description correct , it would have saved me hours on the computer searching .Update: (January, 2019) This product is still going strong well into its second year of use. I bought 4 more in the summer and some are quite a bit smaller than the original but still good for my first graders. The students use these everyday and they continue to hold up.Original: I purchased this ball to use in my classroom because of the legs. So far, so good! Most of the students have had a chance to use it. It is made of heavy plastic and seems like it will hold up to the rigers of classroom use. It is quite small so it's perfect for children in kindergarten, grade one or for possibly grade two if the child is on the smaller side. The only con for me was that it didn't come with a pump. I bought a slightly larger stability ball with this one which came with a hand pump so I lucked out. Be aware that you'll need something to inflate the stability ball.I bought the stability balls for a grade 1 classroom and introduced them to students as a tool to help students stay in the green zone for optimal learning. After 1 week of use, they have turned out to work well for three students and to be too much movement for a couple of others who bounced too much on them or were prone to tip backwards. The quality of the item is good and the feet when used properly do prevent the ball from rolling away when it is not being sat on. Hoping they stand up to the test of time.Very well made. My ADHD teenager sits on it while doing homework and there is a lot of jumping and moving going on. She cannot sit on any other chair without complaining it gives her back pain. The ball seems to have done the trick.It worked well but these balls pop too easily. No one was using it inappropriately but it isn't up to classroom use.Was hard for my son to sit on