At first I found this app a little confusing but the more I used it the more I realized how well designed it was.
The best thing about it is its well-designed A-B repeat feature. As a classical singer, I use it to repeatedly play back small segments within a larger piece, and particularly small segments that are of greater than average difficulty. This helps me master them very quickly. You could not ask for a better tool for that. It is also very easy to change the A and B points as you go.
It basically lets you select any of the songs already found in your devices music application, and then lets you modify their playback in accordance with your needs.
While my primary need is an easy to use A-B repeat feature, it also allows you to speed up or slow down playback without changing the pitch. (A different version of this app also allows you to change the key in which a song plays back, but that one only allows you to load one song at a time, so this is the one to get if the music youre trying to memorize is saved on multiple tracks.)
The only negatives are: (1) the lack of a good set of instructions built into the app, (2) if using it on an iPad, it only displays in landscape mode and (3) I would love it if the repeat feature would continue to work when you are in a different app reading the lyrics you are trying to memorize. These are not major drawbacks though. As to the lack of a good set of instructions, you can quickly learn what you need by trial and error(or by sending an email to the designer).
This has quickly become an indispensable app for me.