The people complaining about the price tag: I get it. But the OP-1 is literally the only piece of music gear you can bring on an airplane and write a whole song. Why has no one else figured that out? With all these groove boxes etc, why has no other music gear taken the step of being portable and all-in-one. Show me another way to write a song in an airplane, and you can have my money.
Apart from...
* Laptop with your favourite DAW
* iPad with GarageBand or Android Tablet with Caustic
* I would put Novation Circuit in similar category though depends on your definition of whole song
* Synthstorm Deluge
* Digitakt Elektron?
* Some of the Roland grooveboxes and systems
* Some of the Akai APC/MPC or Native Instruments Machine? Not fully familiar with either but I thought e.g. Akai Live lets you do a full production
* Bunch of other stuff I'm sure I'm forgetting
I feel we are actually in a bonanza and cornucopia of portable music creation devices! And that's a good thing :). Heck even the Yamaha Reface is super portable, battery powered, real keyboard, lots of sounds, and you can hook it up to phone or a simple recorder/looper to make good stuff on the go. OP1 might be the smallest by some metrics, that may or may not be seen as a good thing.