Chris Schulwrote a review of GoPro Handlebar Seatpost Mount on May 7, 2011
I do really like this mount. It is pretty stable. (After modifying) I mainly use this for my handlebars on my motorcycle or attach it to the passenger's foot peg.
I did modify the mount to a point. I put rubber in the inside to give it better grip. Without the rubber, it tends to roll back on me.