Absolutely great for abs.
Try crunches with your feet against the wall for a change. You can vary the difficulty by changing foot position and your position on the ball.
Then, when you can do it, try side crunches. Make your elbow touch the opposite knee, lift the foot OFF the wall at the same time.
Another great exercise on the ball is the rollout. Go into a push-up position, feet on the ball, then do a reverse crunch and get your knees as far in as you can towards your chest.
I do these constantly, some of the best core exercises there are. It works not just abs, but your core.