Having been primarily an AD&D player in my younger days, I retain a fondness for most of the sub-classes included in the Players Handbook. As I've immersed myself more fully in OD&D, though, I have come to see the proliferation of sub-classes introduced in the Supplements and The Strategic Review as blurring the notion of what a "character class" is. How to reconcile these two positions?
At present, I consider all classes beyond the original three to be "specialists" and, except in the case of the thief (whose status is still unclear despite years of wrestling with it), very specific specialists at that. I've already talked about how paladins into my setting. Druids are a secret society made up of former clerics of Lawful gods, who now oppose Law and Chaos in equal measure. Illusionists are members of an esoteric school of magic and assassins belong to a hidden brotherhood.
I simply don't like the idea of "generic" sub-classes, preferring instead that they all be tied to some aspect of the setting. I feel this way for several reasons. First, it means that, by and large, most PCs and NPCs will belong to one of the Big Three classes. Second, it means that, if a player does wish to portray a member of a sub-class, he's signing on to a large number of "social" restrictions/demands to make up for his character's increased power compared to members of the base classes. Finally, I genuinely think most of the "standard" sub-classes pretty much demand some kind of in-setting context to work. I don't think the paladin or the monk or even the assassin, as written, are archetypal enough to be used without some hook with the setting I'm using. If a player just wants to play a "holy warrior," he can be a cleric or even a zealous fighting man. If a player just wants to be a guy who kills for money, he needn't be a member of the assassin sub-class, which, to my mind anyway, is something much more specific.
So, I do like and would allow sub-classes. I just think they need to be uncommon and bound to the setting better than they are in baseline OD&D.