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The Shadowcaster is a caster (duh) with a very limited repertoire of shadow-related gimmicks that can't come close to any divine or arcane caster in terms of versatility and power. He can't really replace either, having no equivalent of "must-have" spells. Good idea, poor execution, as with many splatbook classes.
I looked at it for a PC character, but even with the fixes proposed by Ari, it just seemed like it was somewhat...broken. It seemed interesting at first blush though.
I've played a couple Shadowcasters, and they are horribly underpowered. Buuuuut, for me they were still a lot of fun to play. It is definitely a flavor class; you're player might have a very specific idea for their character. Again, I personally had fun with it, but as a DM, don't worry about this person breaking the game.
What I found helped a lot, both with mechanics and flavoring, was adding the Dark Creature template, found in the same chapter of Tome of Magic as the Shadowcaster is. It's a +1 lvl adjustment, but gives the caster a lot of powers I felt the class should've had to begin with, and will help make the shadowy stuff easier to actually pull off.