I've got virtually every accessory to go with the GoPro HD, and I figured this would be useful for ...something. My thought was diving/spearfishing. The first one I received was broken (not the housing, but the little black clasp that locks it together). Amazon exchanged it and have now used it a couple of times. It's good enough and does what you'd expect, but I just don't see how you'd get any decent footage with it. The viewing position/angle of the camera is fixed, so once it's attached to your wrist it can only look in one of two directions (depending on how you orient it.)
It would be far better if the housing sat on something that would pivot/turn, that way it wouldn't matter how you attach it to to your wrist, you could adjust the viewing position - e.g., it could look straight down your arm, for instance.
Unless you have a really specific need for this, I'd stick with e.g., the head strap.