I think crunches are really working..
I have a few Abs workout apps separate from this one which really worked. Two caynax abs apps. Depends on the tablet you use. There are short and long exercises but I noticed a difference within 2 weeks and don't have to do crunches. That, and a good diet and you'll be a happy bunny.
you might want to be careful with too many crunches. you should look into crunches not being good for your back. Google it. it builds an uneven tightness in your torso. leg lifts are probably much harder and more effective.you might want some back strengthening exercises as well.