My own sad experience tells me that IK is best used for the legs and when you want a hand placed on something (ie the character needs to climb a ladder or the like). The spine and arms should have controllers for both IK and FK. There is a feature called IK/FK blend which is one of the attributes of an IK handle (and that attribute can be connected via the connection editor to a control object of your choice). This attribute allows you to choose between IK and FK and key between the two.

As a rule, arms (and even spines) are best controlled through FK. One BIG problem with IK on parts other than the legs is that it is very difficult to control with accuracy, and unless one puts a LOT of work into adjusting curves, one will get "floaty" animation.

Hands/fingers are best controlled via set driven key.

Having said all of that, I don't suppose there is any harm in trying IK on fingers. For all I know it might actually work wonders, but I imagine it would work best on a "close-up" arm rather than a fully rigged "general purpose" character. Of course, it would be a right b*tch to control!!!

Hope all that helps!