A party of 6th level characters, including the wizard3/cleric3 who would soon advance as a mystic theurge. He had practiced spellcasting for both classes, so he was casting as a 6th level character, although he only had access to 2nd level spells.
The party ran into a rather large group of monsters. The battle that followed was quite difficult, resulting in a lot of damage to the party, as well as a character death. A single fireball would have made this an easy fight. A prayer could likewise have turned the tide. These spells were still two levels off for the character.
The mystic theurge is going to be a lot better than a basic multiclassed caster. However, he's going to be less powerful than a single classed caster; the additional spells don't make up for the lag on higher level spells.
I don't think that anyone would argue that the MT is criminally weak at the low levels, particularly wiz3/cler3, the absolute rock bottom. To me, that alone makes the class broken. Of course, I also think they're too good at the top levels...
You could tweek the Arcane Hirophant from Races of the Wild for a Psion/Druid, though the familiar companion won't work out quite as well (The Psi-crystal/companion?)