Some people say they're extremely easy and pointless, like half-ass sit ups. THAT'S BECAUSE YOU'RE NOT DOING THEM RIGHT! You have to NOT have your hands behind your head, you have to lift your shoulderblades off the ground, and you MOST DEFINITELY need to lift your legs up.
I can do 100 pushups in one go, with little difficulty. But 50 crunches is harder than that! After I'm done with only 50, all I can do is lie down, and have about 10 seconds of intense searing pain. Pushups have NOTHING on crunches! It depends on how high you lift your legs, the fewer inches off the floor, the better.
About the only thing harder than them is squat-thrust-jumps, but crunches are the unnappreciated and misunderstood workout.
(Just felt obligated to say that XD)