A few things.
First, I wasn’t pleased with the build quality. I know that some people are fine with it, but in the ~9 months I owned it, the power/volume knob split so that it was always loose, the body warped badly enough to crack near one of the module input/output jacks, and the back just never felt secure. The first two were fixed under warranty and I sold it before they could recur. I don’t think there’s a way to fix the back.
The build quality undermined the—for me—primary benefit of the OP-Z: it’s portability. It’s hard to feel good taking it with you if you feel like you’re going to break it by taking it with you. I wasn’t especially hard on it, and always kept it in the TE roll-up case when I put it in my bag. But it still didn’t hold up.
I didn’t really miss having a screen, FWIW. It doesn’t take long to learn to use the OP-Z without one. And you can always use the app if you need it. But I do think it’s too easy to bump the encoders and change a parameter. It sucks to work hard at getting something just right, and then you accidentally bump an encoder and throw off the sound.
For what it’s worth, the OP-Z never made any sense to me for performing. Because of the location of the buttons, you often have to use two hands for things that would take one on pretty much any other device.
And this one’s mostly on me, but I felt like everything I made with the OP-Z sounded pretty similar. I don’t actually have many complaints about the sounds you can get out of the OP-Z. The synth engines are fine. I just seemed to get in a rut with it.
All that said, I miss having a portable, all-in-one groove box. I don’t want another OP-Z for all the reasons I explained, but I do want something that does that. I’ve got my eye on the Synthstrom Deluge, which ticks all the boxes but portability—it’s the size of a small laptop! I’m also interested in the Polyend Tracker, but (a) no battery, and (b) it’s still pretty big. And there are all kinds of iOS apps, but I haven’t found an iOS “groove box” that I gel with, yet.
Anyway, that’s a long-winded response with lots of subjectivity. Speaking of subjectivity, this is worth mentioning: I’m barely over a year into my music production hobby. I’ve played guitar for 20+ years, but I just started in on synths and samplers a little over a year ago. I’m still learning and figuring out how things are supposed to work/how things can work. The OP-Z taught me a lot, but I could tell I wouldn’t care for it in the long run and I haven’t yet begun to second-guess that judgment.