I do look at hands.. but I wouldn't let something as petty as chewed fingernails turn me off of someone. As for my proclivity (I wouldn't quite call it a fetish) towards hands, they're the primary thing we use to reach out to people. Obviously. xD You can learn a lot about someone by watching their hands.
You can tell if they work hard, wash often, play a certain type of instrument. You'll notice if they're strict about beauty and hygiene, or if they're more laid back about their bodies. You can wash the dirt off of your face after a hike, but you can't (easily) remove the callouses from playing the Violin, or moving rocks. It's easier to wipe the tears off of your face after a day at the hospital than it is to repair your recently-marred fingertips.
That, and how a person expresses themselves through touch says a lot about how they feel about that thing or person. The list of words used to describe simple actions involving your hands is immense, with things like "stroke" and "caress" contrasting so violently with "grab" and "wrench".
I never really thought that hands were a turn off but I know that my boy friend sometimes notices my fingernails. He rarely comments when I do something new with them like color and such. But when I do say something about them he always says that he did notice and they look very nice. But then again he doesn't say anything at all if they are not painted and I cut them all off either. I would say that if he likes you then it shouldn't matter. If he is really so sensitive that he is going to nit pick about your nails then that is just mean.
I don't think you should worry too much. Although that is a bad habit, it won't really bother a guy that likes you unless A) It only bothers him because he cares if you are in pain, or B) He has some strange hand-fetish.
True, hands and such are beautiful when small and delicate, it really shouldnt matter. I wouldn't loose sleep over it.
My cousin actually has this same problem, although she has it a lot worse. She bites her nails until they bleed, and she even bites off all her skin on a few of her fingers all the way down to her knuckle. But I am telling you, she has been with a few guys in her days, and is even getting married now.
I also crack my fingers and my knuckles are HUGE. I too worried about if my guyfriend would be turned-off by this, but he simply isnt. A guy will love you for YOU, fingers and all, and if he doesnt...well screw him.