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When using Allen Wrenches (Hex Keys), I've always held onto the long end because I figured it gave me better leverage and torque. Sometimes, in hard to reach places, I would hold the short end.
Then I saw these ball end hex keys. They claim that:
The advantage is that their ball end make it easier to slide the wrench into the receiving slot.
In my opinion, they've put the ball end on the wrong side.
Have I been using these incorrectly my whole life?
The ball end of the wrench fits less securely in the socket than the squared off end so you can't apply as much torque without risk of the wrench slipping or stripping the bolt head. So it makes sense that if your wrench only has the ball end on one end, that it's on the long end so you have to use the short end to hold the wrench to limit how much torque you can apply. That preserves the more securely fitting squared off short end for applying the most torque on the fastener while you hold the long end.
Its ok to use either end of the wrench on the fastener and sometimes space limitations will force you to use one or the other.
There is another whole class of hex wrench to consider as well. These are called Tee Handle Allen wrenches. You can get these in both regular straight cut ends and in the ball end style. For many assembly and disassembly operations they are superior to the old standard L shaped allen wrench. The Tee Handle allows for a reasonable amount of torque to be applied to the bolt or screw. The ball end permits the tool to be used to tighten or loosen the fastener without having to be axially aligned with the hex slot.
. . . 
The Ball end is not designed to apply breaking or final torque it is intended for faster easier removal once loosened.
