A good friend of mine, and the rest of his gaming group, swore it was overpowered because "You basically get free magic items from level one, and you can change them whenever you want!"
Kid. You. Not.
Still haven't been able to convince them otherwise, because they claim it's a virtue of my mechanics mastery more than core.
You're good enough to have a mechanical mastery over core and build characters much more powerful than theirs, but not good enough to know what is balanced and what isn't. Funny thing, cognitive dissonance...
My original point. Non-core is overpowered because they've added to the game. You're Druid is overpowered because you've uncovered some sort of trick. It irritates me.
Back to the original bit, I agree that Dragon Disciple can be a very fun PrC for a melee class. That 'spontaneous spellcasting' thing was always confusing, but a dip into bard works nicely. An often overlooked entry method that's core is a level of Assassin.
Rogue 3/ Ranger 2/ Assassin 1/ Dragon Disciple 10 - 3d6 sneak attack, two-weapon fighting, lots of strength. It's an odd one, but it can work.
[edit]Also, isn't there a Core/SRD guide or compendium around here somewhere? I did a quick scan through the handbook section and didn't see it.