I'm currently playing an 8th level Grey Elf Shadowcaster in a Material Plane Greyhawk campaign. Some of my thoughts:
In terms of roleplaying a Shadowcaster though, I think it's the bees knees.
I can't agree with this more. They're fun to play and they cover areas largely ignored by the PHB wizard/sorcerer.
I have reservations for the class in terms of utility though.
Again, I can't agree with this more. So few spells and, unlike a sorcerer, you can't trade them out. For flavor I understand why its done, but it would be nice to swap out a low level spell that, through play, I have found useless for a spell that I constantly buy wands for.
Also, compare Eldrich Blast of a Warlock with Killing Shadows of a Shadowcaster. At first level, a warlock gets unlimited uses of this spell (provided they take it). A shadowcaster gets 1 use per day of Killing Shadows at 5th level. No comparison. Mind you, at 8 level I have an orb of shadows that I dedicate for using with Killing Shadows, so I have 3 uses per day of it. Now compare that with all the uses of Magic Missile, shocking grasp, summon monster, fireball, lightning bolt, and shout that a wizard/sorcerer gets. No Comparison.
My group has 4 other players: a Vampire (8 monsterous levels), a psion, a cleric, and a monk. Compare to the vampire, the shadowcaster is nowhere near as sneaky, and is nowhere near as tough. Compare to a psion (same holds true for a sorcerer), they can waste their total power points (spells per day) on a single spell if they so need to.
In short, its fun to play something this different and unique, but it just doesn't hold is own (at least by 8th level).