I've been using it for almost two years now, and I'm not going back.
It's based on a spare Blackberry Q10 keyboard and a custom Arduino-compatible board that reads the keyboard matrix and outputs it as USB HID to the phone. From the viewpoint of the phone, it's just a regular USB keyboard, so no special software is needed.
But I do use a custom virtual keyboard to have just two rows of symbols that are not natively on the keyboard, as I didn't want to add another layer of rarely used symbols that I'd have to memorize.
(On the image you can see Ubuntu with XFCE4 running on it. I chose Ubuntu because it's what was easiest to get running in a chroot jail on the phone. I'm using VNC to display the GUI. I even managed to get FEX (x86/x64 emulator) and Wine running, so it runs x86/x64 Linux and Windows apps.)
Yeah, it would be nice. I would also prefer a horizontal version.
That's sadly not really possible, though. The issue is that you need most of the weight in the keyboard part of a horizontal keyboard phone. If you look at any of these, they all have battery, mainboard, cameras and IO in the keyboard part of the phone, with only the screen being on the other part.
With a keyboard attachment, that's not possible. You have all the weight up top. That means, you can never get the center of gravity in a comfortable position.
Believe me, I tried. I only had horizontal keyboard phones before, and the first 8 prototypes where some kind of side-sliding phone, and they all didn't work out, because of the COG. Even if I added weights to the keyboard, that wasn't enough. To balance the ~225g of that phone, I'd have to add >300g to the keyboard, and I really don't want to carry a 0.5kg training weight in my pocket at all times.
So I gave up and stuck a Blackberry keyboard onto my phone :)