Just a quick feature update: I've just added tools to the "Select Grouped" operators (Shift-G) which allows you to select all the objects/bones that are affected by the active Keying Set.
In Object mode, this is mostly useful for selecting all objects that a absolute Keying Set includes.
1) for doing mass prop animation, you'd include all their location properties in a Keying Set
2) for checking which objects already have animation, change the active Keying Set to "Available" and run this operator. This will get you all the objects which have animation channels already, which can have further keyframes added
In Pose mode, the uses for this are more obvious:
1) Select all bones included in an absolute Keying Set to just move those which are keyable
2) With "Whole Character" Keying Set, use to check which bones it will affect
There are probably many more uses for this kind of functionality. But use your creativity!
Back when researching the idea of having Keying Sets for Blender 2.5, I came across the "Character Sets" for other animation packages from the "other" big A.
Those things there are quite comprehensively integrated into the whole way that animation controls are visible, presented in Graph Editors, selected, and/or generally behave. However, I'm also aware that they had some rough edges, where they'd end up being a bit too restrictive on what you could do while they were active. AFAIK, they acted more like "selection sets" for objects, with "marking" properties for keyframing separately based on current selections being the other common workflow.
While this does move things a step closer in that direction, I still don't think it's necessary or perhaps desirable to go all the way down that path. Nevertheless, it's probably cool to have one more neat way of selecting related data, which may hopefully speed up workflows in some cases.