I voted "of limited use"
Here is why:
For a cleric, the feat is worthless. A cleric should be able to turn the undead in question. If a cleric cannot turn the undead (because it is impossible), he she is fighting an undead at a much higher challenge rating than his level. This should not happen very often. The only cases I can think of where this really would happen is with turning resistance and/or the most powerful undead monsters. These cases are rare enough that is not worth spending one of the clerics precious few feats for this one. A cleric would be much better off with heighten turning, empower turning, or quicken turning instead if he wants to spend a feat on fighting undead.
For a paladin, this is a different story. Because the paladin already sucks at turning undead this is a useful option. However, to make it useful, the paladin has to expend 3 feats - which is extremely costly. The paladin will need the prereq extra turning, divine vengeance and then quicken turning in order to spend one of the turn attempts as a free action. Only if the paladin can do this as a free action is the feat useful. If the paladin does goes the route of getting quicken turning that he most certainly should get divine might (or divine shield or both). That is most of the paladin's feats right there thus limiting the usefulness of this feat. While the paladin can be doing up to weapon base damage + divine vengeance + bane (undead) + holy +divine might + spells in damage against the undead he has lost the benefits of focusing on mounted combat and any other route the paladin might want to take to focus on abilities. However, the above example would be dealing:
+2d6 (great sword)
+2d6 (divine vengeance)
+2d6 (bane undead)
+2d6 (holy - most undead are evil)
+1.5xstr bonus
+chr bonus
in damage per HIT not to mention spells, magical bonus of sword, smite, and power attack. And for a paladin focusing on destroying undead I don't think the previous is too ridiculous a case - it would only require the equivalent of +5 total enchantments (I would make it ghost touched as well of course, but that is not strictly necessary).
However, with all that said, this is one of my favorite feats for story enhancement. While the feat is not as powerful as others, it goes very well with the concept of of an undead hunter (no pun intended for the PrC, although that serves as a perfect example of where this feat would be most useful).
Here is why:
For a cleric, the feat is worthless. A cleric should be able to turn the undead in question. If a cleric cannot turn the undead (because it is impossible), he she is fighting an undead at a much higher challenge rating than his level. This should not happen very often. The only cases I can think of where this really would happen is with turning resistance and/or the most powerful undead monsters. These cases are rare enough that is not worth spending one of the clerics precious few feats for this one. A cleric would be much better off with heighten turning, empower turning, or quicken turning instead if he wants to spend a feat on fighting undead.
For a paladin, this is a different story. Because the paladin already sucks at turning undead this is a useful option. However, to make it useful, the paladin has to expend 3 feats - which is extremely costly. The paladin will need the prereq extra turning, divine vengeance and then quicken turning in order to spend one of the turn attempts as a free action. Only if the paladin can do this as a free action is the feat useful. If the paladin does goes the route of getting quicken turning that he most certainly should get divine might (or divine shield or both). That is most of the paladin's feats right there thus limiting the usefulness of this feat. While the paladin can be doing up to weapon base damage + divine vengeance + bane (undead) + holy +divine might + spells in damage against the undead he has lost the benefits of focusing on mounted combat and any other route the paladin might want to take to focus on abilities. However, the above example would be dealing:
+2d6 (great sword)
+2d6 (divine vengeance)
+2d6 (bane undead)
+2d6 (holy - most undead are evil)
+1.5xstr bonus
+chr bonus
in damage per HIT not to mention spells, magical bonus of sword, smite, and power attack. And for a paladin focusing on destroying undead I don't think the previous is too ridiculous a case - it would only require the equivalent of +5 total enchantments (I would make it ghost touched as well of course, but that is not strictly necessary).
However, with all that said, this is one of my favorite feats for story enhancement. While the feat is not as powerful as others, it goes very well with the concept of of an undead hunter (no pun intended for the PrC, although that serves as a perfect example of where this feat would be most useful).