I have an old ipad 2 and while I don't use it as a remote control, I do use these apps sometimes..
GarageBand, KORG iMS-20, and NLog PRO, for the Keyboard sounds, I'm a guitar player and using a small keyboard controller along with those apps lets me get some basic keys/pads and such without needing to buy/repair/maintain and store a half a ton of real vintage keyboards in my living room.
I also use the DM1 and FunkBox drum machine apps sometimes, as well as GuitarToolkit for tuner and metronome, and also Thereminator for the occasional odd bit of awesomeness.