OP-1 has a lot of options that make you feel like you're making music, when in fact you're trapped in an endless round of "What happens if I...?" inside the feature space.
Hence the novelty tag. They're not particularly productive instruments, but they can be a fun way to pass an hour or two.
A software environment like Max, PD, or Reaktor is much less limited. But the learning curve is much steeper, and the instant gratification factor much lower.
And then there are the code-based environments like Supercollider, in which you can implement the OP-1 sequencers and synths with a few lines of code, and easily create your own variations - but the learning curve for non-coders might as well be vertical, and it's not shallow even for experienced coders.