The biggest thing about Mystic Theurge is, that while you get a *ton* of spells, you're behind the power curve, compared to a single-classed wizard or cleric of your same level.
For example, assuming you enter Mystic Theurge as soon as possible (Wiz 3 / Clr 3 / MT 1), as a 7th level PC (MT 2), you'll just be getting your 3rd level spells. OTOH, if you'd stayed as a "straight" wizard or cleric, you'd be getting 4th level spells when you made 7th level.
Most MT builds I've seen use the Practiced Spellcaster feat (often taken twice, once for each of the two original classes)...while this doesn't let you get the higher-level spells any faster, at least it does make sure that the "power level" of the spells you *can* cast is maxed out.