Crunches are great for your abs if you do them correctly. Done incorrectly, they can put undue stress on your neck and back. To avoid injury, be sure to use the correct techniques.
The biggest risk is if you jerk your neck to give you the force to crunch. While doing crunches, keep your hands behind your head and your elbows back while using your stomach muscles to lift your head and chin towards the ceiling.
Once you've mastered this technique, try lifting your legs off the ground or assuming a bicycle position with one leg bent and the other straight off of the ground.