I have seen Doug Marcadia's videos on Funker Tactical and he can definitely make it seem like a formidable weapon.
http://www.youtube.com/watch?v=Sx63kygF1cs I have done some practice with them as well, and I have found that the general use and fighting application is different than a straight edge. When using the karambit in hand to hand, you need to think of it as an extension of your arm, which makes your hand more effective. You don't actually gain any significant reach, but you do have the advantage of an edged weapon in your hand. With a more traditional style knife, holding the edge forward, you get those precious few inches that make the difference between a light incision and a serious cut.
Still an effective weapon IMO, it just requires a different training method than most knives.