Not only are they completely boring, but they also apparently aren't good for your back either. All that flexing puts unnecessary strain on the back. People usually do crunches to tone their abs and get closer to the six-pack stomach, but ironically lots of crunches don't accomplish that. Nutrition and total body exercise (i.e. dropping your body fat percentage) play a bigger role in the ab aesthetic than tons of crunches. Plus, overdoing crunches just overdevelops the rectus abdominus relative to the other ab muscles. The result of a rectus abdominus being a lot stronger than the transverse and obliques would be? Right. Poochy belly. If the other abs are too weak to hold the hypertrophied rectus abdominus in place, you may get that great looking six-pack from the front, but from any other angle you'll have the distended gut.
I stopped doing crunches ages ago mostly because I got bored with them. The last thing I want to do at the end of a workout is 100+ crunches. Now, having my workout partner throw a medicine ball at me while I'm on a situp bench and then proceeding to swear to high heaven while throwing the medicine ball back... that sounds a lot more fun to me. It hits my abs harder and takes a whole lot less time than crunches. Swiss ball planks, leaping push ups, dragon flags, L-seat chin ups, L-seat dips, front levers, etc. are all better choices for hitting the abs. There are plenty of other core exercises which are more effective, efficient, and just plain more fun.
Johnny Kuo is an I-Liq Chuan martial arts instructor and a health and fitness enthusiast. Originally trained as a biologist and then a biomedical engineer, he has been a medical imaging researcher, tai chi teacher, software developer, research engineer, and freelance web developer.