(( VaginaPagina ))

It was almost never right for me, but I have odd luteal phases (formerly 9 days, more recently 16 days). I mostly used them to amuse myself with how off they were. I also have irregular periods, so it never knew when to expect my ovulation either. Which was fine, of course, because I generally had a pretty good idea.
You're better off tracking symptoms -- cervical mucous/position, temperature changes, etc if you really want to know. And if you're fairly regular, I'd wager the calculators would be pretty accurate, because they just average the data you've given them from the past.