I'm got to chime in about Mystic Theurge and Eldritch Knight being boring. I don't give a flying fig about their power levels - overpowered, underpowered, balanced just right; it doesn't matter. I do care that they do nothing new at all. They have no unique abilities, no new powers; they just combine good BAB and spellasting or two lots of spellcasting, and that's it. Where's the individuality, the class abilities that you can't get in core classes? IMHO, prestige classes should be able to do things the core classes can't, not just do more of what the core classes can already do. Loremaster? Very cool. Gets Loremaster secrets, something you can't get anywhere but Loremaster. Assassin? Death Attack, an ability which appears in no core class. The same for Archmage and many other classes. (I'm ill at the moment, so forgive my hand-waving 'and many others' here. They're there, I'm just a bit too sick to dredge them up.) Any prestige class which does something new has a hook, something special about it. I love Spellsword, for example. It's characterful and interesting. Mystic Theurge, Eldritch Knight, Cerebremancer et al? Boring, insipid and tiresome, highlighting the worst aspects of prestige classes. If you're going to make a prestige class, for gods sakes do it properly, don't just bodge together two bits of core class mechanics and pretend you've done something cool.
Synch.