I haven't had a chance to use them in a game yet, but I'm working on campaigns where they're primarily used.
One campaign is a 3.5 conversion of Lankhmar, which the generics work perfectly for. However, I'm also using a lot of other UA rules for this game since it's one of the settings that defines classic, low-magic, "sword & sorcery."
Another campaign is based off of a setting used in my fantasy fiction. Standard D&D (in any form) never really cut it for my settings, though I'm a big fan of the game (and other game systems had I lot of elements that I didn't like in them). It pretty much uses the standard generic classes as-is, though I'm altering the standard races, & creating setting-specific PrCs.
Though you could feasibly create all of the class abilities as feats (just with strict prerequisites/feat chains), I think that some class abilities should remain as class abilities, & only be available through PrCs made for the generic classes. A generic unarmed combatant seems great for generics, but the mystical, devoted martial artist ala the Monk core class, IMHO, seems best suited as a PrC for the generic classes (just have a generic PC follow the unarmed combatant path as a warrior or expert to gain access to such a PrC).
I did make up a couple of unique feats/flaws for the generic classes. They're 1st-level character feats only, so they can't be taken later at all, or multiple times.
New Feats for Generic Classes:
Increased Skill Points: Increase the character's base skill points for her class by 2. (Warriors and spellcasters go from 2 to 4; experts go from 6 to 8.)
Restrictions: May be only taken at 1st level. Cannot be taken multiple times (i.e., it cannot be selected again if a character multi-classes into another class or a PrC).
Increased Hit Die: Increase the character's base Hit Die for her class by one die type. (Warriors go from a d10 to a d12; Experts go from a d6 to a d8; Spellcasters go from a d4 to a d6.)
Restrictions: May be only taken at 1st level. Cannot be taken multiple times (i.e., it cannot be selected again if a character multi-classes into another class or a PrC).
New Flaws for Generic Classes:
Decreased Skill Points: Decrease the character's base skill points for her class by 2. (Experts go from 6 to 4.)
Restrictions: May be only taken at 1st level. Character must have a skill point base of 4 or higher (before adding racial/Int modifiers). Cannot be taken multiple times (i.e., it cannot be selected again if a character multi-classes into another class or a PrC).
Decreased Hit Die: Decrease the character's base Hit Die for her class by one die type. (Warriors go from a d10 to a d8; Experts go from a d6 to a d4.)
Restrictions: May be only taken at 1st level. Character must have a base Hit Die of d6 or higher. Cannot be taken multiple times (i.e., it cannot be selected again if a character multi-classes into another class or a PrC).
I think these feats work rather well with the generic classes: it allows for a bit of variety for characters.