I have one! It broke, though, and it's been a long and horrible process. That aside, when it was working, I was using it for about half an hour or so between 3-5 days a week and I noticed a positive change in my butt, legs, and even my tummy. I would recommend staying away from the manufacturer called "V-Fit" as this is who made mine, and the first one broke within 60 days, so I returned it to the store I bought it from and they replaced it for me. The second one lasted a little longer but it broke last week also. It said the maximum weight was 100kg and I weigh 73 kg, and last year I only weighed a few kg more than that as I am just now beginning WW, so I really do blame the manufacturer for its lack of durability. It was also so incredibly loud! It squeaked like you would not believe. Nothing I did made it any quieter, either.
That said, I really like the size of a mini stepper, and have just ordered a new one from Amazon after reading a ton of reviews on them. It is the Body Sculpture Twist Mini Stepper or something like that. I paid £29.99 with free shipping, so about $60 Canadian (or American, near enough, I just happen to be a Canadian living in England so automatically convert to Canadian dollars!). I definitely recommend them if you are looking for an inexpensive piece of gym equipment that you can put under a bed, or in your case, a desk!