The chicken pox vaccine doesn't give 100% immunity, but if one gets it later, it's most probably a mild version. Btw, one can actually get chicken pox several times, so that isn't 100% sure either, although it is pretty rare. As I have understood it, they are not sure how long the chicken pox vaccine will last, so it might be prudent to re-vaccinate later on.
I absolutely hated having chicken pox as a kid, so we are going to vaccinate our daughter (have just been a bit slow about it...

). And then there is of course the added benefit of not getting shingles as an adult.