I've been playing in a game with a Mystic Theurge. Characters are 13th level at the moment. And, I gotta say, I'm not particularly phased by his presence, as the party Cleric. He can't fight in melee, can't cast Heal, can't cast Heroes' Feast, can't cast Greater Scrying, etc, etc. He can't turn undead to save himself, either, but I'm a bit PrCed away from that, too
To be fair to our Mystic Theurge, he's played in a more tactically-minded fashion than the party wizard, who, for all his incredible cosmic power, fails to get out of harm's way and chews up a lot of my hp, via the wonders of Shield Other.

(Which shall henceforth be cast by our party tank - thankyou Imbue with Spell Ability!) The workmanlike attitude may have something to do with my lack of hostility, though in all honesty, it doesn't take away from my perception of my character's party role to have someone else capable of shedding low-level spells to patch people up. It's actually quite liberating to not have to do all the healing - and there's usually a lot to do in one go, as our DM tends to aggregate the combat into less frequent, more demanding fights.
Our wizard player still moans and bitches when we have a couple of non-combat talkie sessions in a row. (There's times when I just want to say, "Back to the Academy, schoolboy, the outdoors is too dangerous for the likes of you."

) But he's never complained of having his limelight stolen by the Mystic Theurge.