>all prestige classes ever officially printed would never be worth taking.
That was certainly their intention for the most part. That prestige classes wouldn't really be an upgrade, but that doesn't mean there aren't some very powerful prestige classes out there.
Magaambyan Arcanist PrC is a massive upgrade to the Wizard, for instance. Bloatmage, Pathfinder Savant, Daivrat are also very solid picks. Agent of the Grave is good for necros.
Clerics benefit from a 1 level Dawnflower Dissident dip and taking Veiled Illusionist.for illusion spells.
Demoniac PrC is also extremely powerful for divine and arcane casters.
As for martial classes, they should generally multiclass back and forth like fucking crazy. Barbarians can single class, but past level 12 it's better to exit Barb.
For martial prestige classes, it's usually worthwhile to dip 1 level of Living Monolith. Hellknight isn't bad either. And Pathfinder Field Agent PrC has its uses as a 1 dip for a Rogue talent.