I had one of these ... twice. Once in the 70's and again a few years ago.
The one I had in the 70's was new. It looked cool in the box, so I was tempted. On a bike it looked dorky (see how many you can find in old race photos). It also looked like an accident waiting to happen. So I got rid of it.
FF to 2007, and I was tempted again. Got one cheap on ebay.
When it arrived, it was impossible to disassemble. The aluminum on these is SOFT. It deformed around the clamp causing the extension to seize. After some very careful filing and tapping with a wooden hammer, I was able to get it apart. But it would only go back together after some more filing.
The design is solid. Once clamped into place there is no play in any direction. Thats at the cost of no fine adjustment. There are notches in the back of the extension that limit its positioning. This is a very poor design in my book.
"Major Taylor" got it right with his square cross section: Infinitely adjustable, solid, with no movement once clamped. I rode a Nitto-made Taylor-type stem, cross-country in '76. It never loosened up unintentionaly, and enabled minor adjustments. This came in handy whenever shoulder pain developed.
Amir Avitzur
Ramat-Gan, Israel