I have a very good friend that is a chiropractor, I'd never tell him, but many of them are nuts. I don't think a few adjustments a year is going to hurt, but when they start pitching for you to show up every week for an adjustment, I start worrying.
Personally, I don't use them any more because the last few adjustments I had seemed to worsen the problem I went in to have corrected. Other people I know swear by them, but I suppose it's a personal choice.