JFit Disc - any reviews?

I am thinking about getting a balance cushion/disc for my chair at work since I am at my desk for 9 hours a day. I have been reading that this strenghtens your core muscles and tones your abs. I was seriously considering a balance ball at first, but then came across the cushion (which is much less obvious than sitting on a huge ball).

I cam across the Jfit disc and a few others.

Any opinions, suggestions on which ones, etc.?

Thank you!