Personally, I tend to envision using binders in a manner similar to their inspiration - pacts / summoning of fiends. On the other hand, I would prefer if the entire class were reduced to a talent tree or series of feats, so that any class might more readily make use of it.
I can see a fighter making a pact to increase his strength, vitality, etc, a roguish class perhaps wanting a bit of fiendish luck or stealth or the ability to see in complete (and even magical) darkness, and a wizard or sorcerer taking a chance so as to achieve a bit of greater power earlier (one spell slot of a higher level than they can currently cast, perhaps, or maybe an extra spell per day at their highest level, or something like that). I can imagine priests, having been forsaken from their deity for impious acts or heretical leanings, making use of pacts to regain some of the power they have lost.
There is just so much potential for the concept of binders, but I just wish it were whittled down feats or a talent tree so that any class could make use of it without losing levels. Imagine constructing an NPC that will interact with the party of PCs several times before finally betraying them. From his skills and class abilities, the party thinks they have a good measure of what their new foe is capable of - and then he makes use of something completely unexpected (due to some odd pact choice, perhaps). Granted, the same can be accomplished with a bit of multi-classing and non-use of one of the classes of the multi-class until the betrayal, but this way it could be slapped on much more easily - just two or three feat choices, perhaps.
Ah, well, I admit it would be a bit complicated to make binders in this manner. (After all, what constitutes the gain from the first feat taken? If it can vary so greatly, it will likely end up as a list of options or suggestions - which would further complicate things.) It's a nice idea, however.
