I've been using this app for three months now and really like it. One feature that I haven't found on other apps is the ability to input a future one-time payment (such as a bonus or tax refund) in the month you expect to receive it. The app then applies it based on your selected criteria such as highest interest first, lowest balance, custom, etc.
In addition to offering the one time payment and typical extra monthly payment amount, you can also set up an emergency savings account that takes a percentage (you decide how much) of your extra or "snowball" money and sets it aside. You can select the month to start that and it shows how your savings snowball, too.
The only reason I didn't give this five stars is that once the month is over your numbers disappear, so there's no history of progress. You also can't start your emergency savings with an amount you've already saved, it starts from zero.
Those two items are fairly minor for me, though, and I like this app better than others and I'm definitely seeing progress in debt payoff.