I have both a half case (bought on eBay from a Chinese maker - Mr Zhou, a very nice and obliging chap too who sold at a very competitive price) and an original Leica grip for the M8/M9.
Obviously the two items cannot be used together but I do swap them out from time to time. I love the look of the M8 with the Leica grip and it is nice to hold in its own way too. But I particularly love the soft but secure feel of the half case in my hand - and the knowledge that it provides an extra layer of protection from bumps and bruises.
I could imagine putting one of them away in a drawer and being unable to find it when I want it. I do this kind of thing all the time with camera gear and I have now learned the need to stay calm as experience suggests they always show up - after all, such things are unlikely to be lost in the field, hence they must be somewhere at home. (I hope).