The player in my group, who had the Mystic Theurge, makes a new character now.
I think he's overreacting, but well... I guess the time before Practiced Spellcaster and 3rd level spells were available has brandmarked him too much, and I think he wants a more
active role, too. So, whatever, as long as everyone has fun (this rules out psionics, of course

)...
No, the Mystic Theurge is definitely not overpowered. That's really fairly easy to see, once you know how tough a loss in spell levels is for a pure caster, no playtesting required there, really, and I never understood the outrage in the first place. People just think too narrow... spells = powerful, more spells (but lower spell levels) = more powerful = wrong.
At very high levels, and with Practiced Spellcaster, I can see the class to be fairly powerful, even more powerful than a pure wizard, but the lack of proper advancement past 16th level is a big downside there. Still as soon as the character picks up 8th or 9th level arcane spells, the plethora of divine spells, up to 7th level isn't bad at all, are worth a lot more than the few more spells/day a pure wizard has (judging from the standard cleric/wizard combo, which proceeds to increase arcane casting ability after 16th level). At this point the class becomes really powerful. Until then, the Mystic Theurge will always be behind a pure cleric or wizard, but as a supporter, the huge amount of different spells is pretty cool, indeed, tho you need Practiced Spellcaster to make them useful. You just need to know where your place is and be happy with a more supportive role, I guess.
Bye
Thanee