The one he's got is different than the others in the thread, but it's a similar idea. A friend of mine had onee ffor a while. The two sides don't really flex seperately - they're on a fairly rigid base thing. The one that courier's using has closed-cell foam pads with no cover, with an adhesive back. The saddle comes with a couple sets, and when they're worn out you pull the old pads off and stick on the new ones.

The argument behind them is to get all weight off the taint and the boys, but as everyone else has mentioned, it just pushes you forward onto your wrists.