I like them, and almost all of my PCs get at least one eventually. The most common being Collector of Stories, both for knowledge whores (or "pokedexes," as we also call them) and those that take at least a single rank in all the knowledges so they can at least attempt a check.
Not sure what are some good "combos..." I'm doing a gestalt game right now and one of the NPCs was a gestalt with Scarlet Corsair prestige class. She knew some Tome of Battle maneuvers and could use Mithral Tornado (attack all adjacent enemies and +2 to hit on all of them as a standard action) once per encounter. She also had +3d6 SA, and the Scarlet Corsair ability to feint as a free action every few rounds.
So...she moved into a good position, used a free action to make a bluff check. Used the Group Fake-out skill trick to attempt it on all 8 enemies around her, won all the bluffs...and proceeded to attack each of them at +2 to hit and for 3d6 SA damage. I'd call that a pretty awesome combo, even though only once/encounter.