For example all the third party podcast apps make a real mess of my podcasts. I can't easily do tests and there is no really decent hand writing app for android.
I think if someone made an app that allowed you to study all the popup chinese stuff (or just selected stuff) on the go it would be a massive success.
Just look at how well Skritter is doing on the IOS.
Popup Chinese is definitely the best service I use but because I spend so much of my time traveling I dont get the best use out of it.