After ongoing testing of the head-rest v.s using the binoculars without it installed (I've got a 10x42 NL), I don't think it helps all that much with image shake. I can barely see any improvement. I imagine this would be even more so with the 8x42 model. Perhaps it'll be more useful with the 12x?
I think it was a case of placebo effect. After spending the $$ on one, you'll desperately want it to work, and your mind will do everything to convince you of that fact.
What it does do well, however (and I think this is what it was mostly designed for) is provide extra comfort during prolonged viewing sessions. It may not be a big factor if you're into viewing fleeting birds, but if you're into more static stuff (larger wildlife, or astronomical subjects) where you may be staring at a spot for hours on end, the head rest allows you to spread the pressure of the binoculars onto your forehead, so that your eye sockets aren't taking the brunt of it. The headrest does this very well, and it's well worth the $$ in that sense.
Sure, you can definitely live without it, but every time I've taken the headrest off, it feels kinda weird and I've found myself putting it back on again.
In summary - it's best suited to people who will have the binos glued to their eyes for prolonged periods. In that regard, it makes a noticeable difference in comfort .