I have an OP-1 and the killer features for me are _extreme_ portability, like even more than my laptop, and the best battery life of any device I have ever owned. I have it with me at almost all times, and I can bust it out and start playing basically whenever. I use it at the beach, on hikes, in cafes, wherever. There's no way I'd be lugging an MV-1 around with me in those circumstances.

I also do like that the OP-1 forces a "keep moving" workflow with it's stripped down synth engines and 4 track recorder. People criticize that, but if I wanted a device with tons of tracks and unlimited sounds I would just use my laptop.