Of course it would be popular -- it's a totally new concept. I mean, a PrC like Arcane Archer is nice but you have it, the Deepwood Sniper, the Peerless Archer, the Initiate of the Order of the Bow, and that's only counting WotC-classes. I won't count all the cleric-fighter multiclass, especially when considering that the Paladin and the Ranger are somewhat multiclassed base class.
So, yes, I would play it, out of curiosity.
That said, my natural preference would go to playing a pure wizard, and my powergamer instinct would go to playing a pure cleric.
Had I made this poll, I would have posed these options rather:
1 I would not play a Mystic Theurge, I'm not interested.
2 I would not play a Mystic Theurge, my DM would not allow it.
3 I would play a MT, out of curiosity.
4 I would play a MT, because I think it's powerful.