I've been on dilantin (generic) since 2001. I haven't had too much trouble with my teeth/gums yet. One time last month I had a popcorn hull stuck between 2 back teeth. Hurt for days.
One thing I do that might help is I rinse my mouth out with listerine almost everytime I go into the bathroom. I brush with a very soft toothbrush twice a day. My dentist gave me children's toothbrushes to use.
I still have all of my original teeth. They're a little smaller than average in size, but all work fine.