I like this app. I got it on sale but I think its worth the full price of $3-4. i have a logitech thin BT keyboard and ipad mini retina. I tried this out with a couple synth apps and it works. I dont mind the foreground limitation but it would be good if it could recognize chords. However as an emergency back up when you dont have a keyboard with you, its a good option. Like if you just bought a new music app and want to play it a bit but dont have your controller with you, this will fill in. the different key layout options are nice and each has its own settings which is also nice. so you can map the "pads" to a drum app and the keyboard to a synth app. I think this is a pretty good app to have in ones iStudio arsenal.
plexuss about Double Decker, v1.0