We use 9png mainly to reduce the size of the apk. But, I am facing a peculiar problem. I am not sure in which folder I have to place the 9pngs in. I had put them in HDPI folder. Since Android is 'clever' it looks like if I use the app in an MDPI phone, it 'scales' the 9png which creates crazy effects. I didn't face any major problem until I tried it in Galaxy note. Looks like the 9png was taken from HDPI and scaled (Note has 800x1280) and it created weird effects.
According to Romain Guy's answer on Google groups:
The behavior of 9-patch is undefined when shrunk. In practice you will see overlaps of the various sections of the 9patch. Don't do it :)
From what I noticed in my projects, a 9PNG is scaled down just like any regular PNG, so I am usually requesting to our designer the entire set of images for ldpi, mdpi, hdpi and xhdpi, and I transform them all in 9patches.
That, unless there is a very simple 9patch, like a plain button background, that makes no difference when stretched or shrunk, so I usually place it in the regular drawable folder.