Core class, with modifications. I still think it's a good concept for a core class, it's just not a very clean concept.
The problem for me was always, why is Paladin a core class but Blackguard a PrC? Why should you have to be a good Paladin first to Fall to the best abilities, when in all fairness it's just as likely for an Evil god to start people off as their anti-Paladins? And how about anti-Falling? If I start as an evil Paladin and get "redeemed", do I gain more abilities than someone who was always good?
Then there's the other Paladin-like PrCs (Holy Liberator?); you can't multiclass with a Paladin, but if you take one of those you effectively can.
So, I reworked the core Paladin to actually be four sub-classes. Guardian (LG), Crusader (CG), Blackguard (LE), and Shadow Knight (CE). Sort of like how a Psion has 6 sub-classes; you can't mix them, each has some slight differences, etc.
It's made things a lot easier to manage.
I also reworked mounts, but that's a different topic.