I bit my nails from as young as I could remember until a year ago, when I stopped smoking. Somehow, without meaning to, I stopped biting my nails too. I then started smoking again, and started nailbiting again, but later figured if I had stopped before without intending to, it couldn't be that hard, and I was right! I just stopped. It's amzing how fast your nails grow, which gives you incentive not to start again...
I did start again but found it easy to stop, and have not bitten my nails now for a few months (still put my fingernails in my mouth occasionally and pretend to bite them, but don't) but couldn't help feeling it was such a waste when I had to cut them for the first time.
Dh bites his nails too, but I have never seen him do it, only his fingers look awful. He must do it in secret which I never did... urgh. Remember often biting my nails so far down they hurt and couldn't bite any further so had to pull the nail off which was very painful. (sorry if it's too graphic!)