Even when used in an environment it was never intended to be used in, my Mystic Theurge is not abusive.
We are using D&D 3.0.
I have a 20 int and a 24 wisdom, I am 12th character level (3/3/6).
I have the feats improved initiative, improved counterspell, reactive counterspell, extended spell and persistent spell.
Our characters were originally converted over from 1st edition where my character was a 9/9 cleric/magic-user. Because we were originally 1st edition characters I have CONSIDERABLY more resources than the normal 12th level character would have. At one time, I had as much cash on me as my character was supposed to have total equipment.
With all of this in mind...my character is still not abusive. I am extremely effective as a character, but not unbalancingly so. Were I to have my normal equipment levels and be using 3.5 spells I would be on the low end of the power scale, but still in the acceptable range.
As is, I am effective without being unbalancing. Our 12th level Barbarian/Fighter...or our 12th level Arcane Trickster ... or any of the rest of our party are all effective characters. I do not significantly outshine any of them in terms of mechanical affectiveness (some I don't outshine at all, like the barbarian/fighter).
Now that is purely my opinion...I am sure some people will read this and be completely disgusted by my character. (Of course...I didn't ask you, you know who you are!).
Cedric