I can do plenty of sit-ups and other ab exercises. I also have a house-mate who is a personal trainer so I know I'm doing them right.
However, I can stick to a routine of doing reasonable quantities of abdominal exercises every second day. But I usually loose interest after 3 - 6 months as I never see any results.
My abs are definitely stronger, however I think I have a hereditary beer gut. It just doesn't want to go away.
I also combine these exercises with a reasonable diet and other exercise e.g. swimming, jogging and other team sports.