Broken: Blender 2.69
I hope, this bug doesn't already exist but didn't find something similar in the bug tracker.
Drivers seem to work only when the variable is set to the transform space on any transform channel of a BONE (seems to work well with objects like empty for example).
If the space is set to world or local, I experienced a lag in the viewport when manipulating the driver bone.
Exact steps for others to reproduce the error
1- create an armature (bone 1)
2- duplicate the bone (bone 2), and set a copy location constraint with local or world space.
3- duplicate bone1 once more (create bone3) and add a driver (average value) to one of its channel (x location for example). Set the variable to :
- transform channel
- ob/bone : Armature, bone2
- type : X location
- space : world (or local both give the same result)
Now move bone1 in x direction, bone2 and bone3 follow but when using ESC or right click, bone1 and bone2 go back to previous location but bone3 stay in place. Only another action will make it (bone3) goes back to the right location.
The behavior seems to be ok with objects other than bones though (same test with 3 empties worked).