Thats all I do for upper abs and serratus. If done correctly, nothing and I mean NOTHING builds the upper abs better. The key is not going to the floor. Stay completely erect under the cable and crunch slightly bending at the upper ab only (about 4 inches max). Hold and squeeze for 5 secs per rep. So during the whole motion, your belly button (and below) never ever moves. It is fixed. Your upper ab development will explode and it only takes 70-80 lbs max to burn em out since your using no body weight to pull it. Only abs.
Not to sell it, but look at my abs in my avatar. Trust me, do it this way and you'll never do a silly useless situp again.
Good luck.