Hey Nee
I assuming you are referring to it's supposed status as a 'negative calorie' food ie that it using more calories to eat and digest it than it contains.
Hate to burst a bubble but negative calorie foods are a big ole myth. Although it is low in calories. I believe it's something like 10% of your calorie intake that is burned through the process of eating and digesting food, whatever food that is. So if celery is 10kcal (dunno just guessing on that one), you'll still be left with 9kcal at the end.
Take into account that if you are calorie counting and have worked out your calorie intake usin your BMR, then that takes into account the amount of calories you use eating, breathing, generally living.
Sorry bout that, still if you like it it's certainly not a bad food to munch on.