I have a player in my present campaign who has had it for a while. He's working toward a spellfire Channeler Prc I devised becuase Spellfire in nature (not mechanics so much) is different in my world. He is one of only 3 people who have it in the world. It is passed down through the ruling family and can only be passed from generation to generation y the king/queen. So even though his brother and sister have it (they are captives right now becuase they are spellfire wielders) they cannot pass it on to their children unoess they become King or Queen.
As far as how it runs in my game. He has never overshadowed the group in terms of raw power. Roleplaying wise he started out not knowing what he had so didn't use it for several levels and is just now starting to use it consistently. It is usually one good burst during a fight and he could do the same with a wand. As far as absorbing magic he very seldom if ever did. Its wasting and action or potentially wasting an action. He absorbed between fights from party casters. He also uses it to heal a fair bit when those last minutes saves have to be done and the cleric is tied up or can't make it to save someone.
The only changes I have made, both of which were quite recent, is that he cannot absorb over his con and the healing is not considered positive energy by undead. The first becuase I never liked the rules for absorbing too much as presented. There isn't a real penalty for having like twice your con but that can add up to a lot during a single fight. It was extra rolls and extra hassle to me.
The Spell fire channeler class he will use allows for him to start storing more as he gains levels in it, as well, as increase his healing and do some other neat effects he wanted. Since he is most likely the only one to ever take this class I tuned it toward what the player/character would want to do with his abilities. they are mostly support type powers.
Hope some of this babblingis useful. I don't think it is to powerful but bears watching.
Later