OK, here's my second post (at least the first one was about Macs).
I got one of these balls when I was in massage school because the instructors seemed to think they'd work better for people who were moving around but needed to sit at the same time; plus, they were supposed to be good for your posture and I do have a bad back. But I found that it rolled *too* much and was really hard to control. This would probably not be a problem if you had limited space around your desk. Then the problem would be, as someone else mentioned, whether it fit. Unfortunately, because they're circular the height of the ball (and of the seat) is also the diameter. But they do cushion the low back well - I actually had physical therapy using one - so if you have back problems a ball might help.
As for the kneeling chairs - I've had *many* of those; there was a time when that was all I could use without pain. They do help you sit up straight, but they have some disadvantages. One is, they're not great if you have bad knees; that's where all the pressure goes when you're sitting in one. The other is, some of them have weight limits. I bought one online and when I was assembling it noticed a small sticker on the bottom of the knee pad that said "weight limit 200 pounds." There was nothing about this mentioned in the online description. It was OK for me, but when I finally sold it I made sure I put that in the ad; I didn't want to be liable if someone bought the chair from me and it collapsed under them. So if you do buy one of these make sure you check that out. (Also - I don't personally think the backrest is worth it; I'm not sure how anyone could use it when they're sitting in a chair like that. Same with armrests.)
I hope that helps.