Now, I'm not saying that breaking it is so easy that we have to take care not to, nor that it is even possible. I personally like the mechanics (though I know how much they suck). Also, I understand that this class caries the implication that it should be used with fixes, which I agree with and that all people should use it with (
hereWizard's Expanded Class article, which just has stuff about the FR's shadow weave feats.
Things to focus on for optimization:
PRC's [Can it work with ScM?]
Metashadow Feats
Other Feats
Race
The shadowcaster's features, so no candle of invocation or things that are independent of the class's mechanics.
Whether or not the features of the shadowcaster should be focused on or whether it's best place is to complement another's role, such as gish or whatever.
I'm simply at a loss on how to compete with a Tier 2 or even 3 with this. I'd like to see this class able to fill the role a party's sole magic user.
Tangent
[spoiler]
I ran a beguiler with the shadow casting fixes replacing the beguiler casting, and it was pretty fun, which is why I don't think this is a worthless mechanic.
[/spoiler]