When I was young (and already thin and didn't know it, stupid me) I took something similar. My caution is that one's body being bombarded with fiber is quite a shock to the system sometimes. You could feel gassy or bloated and it's void of nutritional value other than the fiber itself.
In short, feeling full is not the same as getting good nutrition.
What has been proven, though, is to change the kind of bacteria in your gut by cutting out most processed foods and eating lots of fresh veggies and fruits. This actually will help your good bacteria in the colon to more effectively digest foods and to help your metabolism as well. Those good bacteria LIKE fruits and veggies as fuel, and that not-so-great bacteria which actually helps to keep us heavier is better fed with processed and starchy foods. It *doesn't flourish* with the healthier fresh fruits and veg. Isn't that something?!
All that to say, an apple or stalk of celery might have more calories than just fiber, but it's better able to help your body in the long run than those pills. Good luck! And I think it's much better to get fiber in foods, the way B suggested. Blueberries, flax seed, psyllium seed, pears-- all of those can go into a smoothie if you are really wanting fiber. Whole veg and fruit are the way to go!