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.)
And here all I can cynically think is "Great, finally a way to make the FP4 a bit more unwieldy and hard to hold with small hands, that had always been way too easy before!"
It's totally stupid. About every woman I know complains (and rightfully so) about these micro pockets. Still nobody in the clothes industry can't be bothered to just put a few centimeters of extra cloth onto the pockets to make them actually useful.
Either no pockets or pockets that are ridiculously shallow and narrow. Like I (M) can fit my whole hand up to my wrist with lots of room left over. Women's pants can fit just their four fingers if they're lucky.