honestly, its all about how hard you want to work them.
If you do 15 situps on an decline bench with your hands behind your head, elbows out, BACK STRAIGHT, and really squeezing your abs to pull you up, you will feel it.
too many people just do crunches for a certain amount of reps, as quickly and effeciently as possible. this is wrong, you want to do them slow, and not use any momentum at all, while squeezing as hard as you can.
Abs are IMO the most important muscle group to developp the mind-muscle connection with. you really want to feel the muscle working through the whole range of motion, and always squeeze hard at the top.
My routine is as follows(pm me for detailed descriptions):
3x15 Cable crunches
3x15 weighted side bends on hyperextension machine
3x15 decline reverse crunches
3x15 Barbell side throws
i dont do all on the same day...infact i only get through about 2 cycles of this per week, but it does work really well.