I've actually done some flintknapping, so here goes...
Obsidian and glass are pretty much the same, with the exception that with obsidian, you have to deal with a cortex usually (the rough, weathered outer area) and obsidian can have natural imperfections in it (weathering, etc). I'm not sure, but at least on the covers, the glass knives look like just broken pieces of glass. These would shatter fairly easily. If you've gone through the trouble to flintknap an obsidian axe or knife, however, they'd shatter less easily. The way you'd make the weapon would reinforce the edge and the body as a whole (bifacial flaking). The edge might get roughed/chipped, but retouching it wouldn't take long, even if you didn't have the skills required to make a knife yourself.
As far as unusual shapes go...the Classic Maya took flintknapping to a very artistic level. I tried to find some good pictures of obsidian eccentrics, but wasn't very happy with my results. Often very small, I've seen things over two feet long with the curves and delicacy of doilies. Here's a link to a picutre, though not as impressive as I would have liked.
A lot of them are simpler than this, but it gives you an idea. When you think about how it was made (carefully hitting the obsidian with another rock/pieces of antler), I think it's amazing. Archaeologists today can't recreate this sort of thing yet, thought they're getting closer.
http://exchanges.state.gov/culprop/elsalvad/fi/0000006d.htmI don't think Reen's would have looked anything like this, but it gives you an idea of what can be done with obsidian. We tend to think of obsidian as crude, I think, because in the Western world the technology was abandoned in favor of metal before it ever reached this kind of awe-inspiring sophistication.
