Once the initial cuts are made, there is nothing better than a stone knife when it comes to skinning a deer. If you don't care about cutting the skin, then a steel knife works fine...but you can't go nearly as fast. Now, if you're like me and you plan to brain tan the skin, you don't want any slices in the skin. A steel knife is actually too sharp (when it is sharp) and has a tendency to slice the skin, but a stone knife is finely serrated and it is the best because it will separate the connective tissue between the skin and the meat without actually slicing into the skin (unless you REALLY bear down on it). I once skinned an elk with an obsidian knife and the guys who were watching me were speechless....I was just RIPPING that knife through the flesh, and I was moving FAST. They were way behind with the steel knives, and I got that skin off that elk so much quicker than they did....and I had a better product because it had no cuts in it!
Just like everyone said, you can't use it as a pry-bar or a screwdriver because you'll end up breaking it in half or crunching the tip off. You have to use a little bit different technique with stone, but it works amazingly well. Those primitive people were not at as much of disadvantage as a lot of people think.