I have to agree: they are a bit too flexible to user alongside the core classes. OTOH, some players may feel that the generic classes may be too lacking for the type of character build that they want; selecting a rogue is better that trying to make a rogue-like character using the Expert generic class, mainly because rogues have more class skills & a higher skill point base than the Expert class (the Expert can select most, but not all, of a rogue's class skills, and won't be able to spend as many points on skills [considering that the rogue & Expert have the same Int score/modifier to skills]). Playing a generic Spellcaster with divine spells is best suited for multiclassed generic-class PCs, since Spellcasters begin with proficiency w/ 1 simple weapon, and no proficiency in armor or shields. You'll notice lots of little details like that when comparing the 2 against each other.
However, on their own, generics are great. I think that these classes would work better with concepts/settings that don't necessarily fit into a standard D&D mold (to a degree, that is). With the generic classes, you could essentially create a barbarian that isn't a berserker, a priest that isn't a warrior-cleric or nature-steeped druid, a skilled character that isn't a rogue, mystical woodsman, or wandering musician, etc. However, using these generic classes will take a bit of work, since you may want to convert over NPCs & other characters into generic classes as well (otherwise they may have a distinct advantage/disadvatage over the generic classed-PCs).