Simple, and very effective.
The worse reviews are just because of the difficulty. You can make it easier or harder just by pressing the star at the top right and choosing a level. I agree with one review saying that after the quite frequent rest days, it adds 14 seconds to your time to plank, which IMO is a lot. The app doesn’t have any glitches and the ads are fine. I like the little reminders they send you. Only complaint is that the rest days are quite frequent and there is no way to change them. I think all it needs is some more customization (like being able to add or subtract time to make it easier or harder, or being able make the rest days more or less). Either way, the app is great, and I would recommend it to anyone who wants nice abs!!!