Almost daily I'm asked- "how do I lose weight from such and such area?"; usually stomach or arms. If you're looking to lose some fat from your mid-section then sit-ups should be way down on your list. If they were any good, then we could scrap all the fancy equipment you see in the gym and replace it with nice mats to lay down on. Unfortunately, losing fat from your stomach, arms or anywhere doesn't happen by targeting that particular body part.
With fat it is last on first off. Wishful thinking will not make it go away from just one area. It amazes me when someone thinks they can build their arms by exercising them directly, and reduce their stomach by using the same approach. This is basic physiology, yet even the most intelligent of people fall prey to this illogical thinking.
If you're looking to lose fat, get your diet in order first, then exercise, don't focus on ab work alone.