Yes I’ve been using it in some form for about 15 years - FYI it originally started as the MessagEase keyboard; Thumb-Key is an open source replacement (maintained by none-other than Dessalines of Lemmy).

The reason I chose it is because I use Dvorak on desktop, but Dvorak is notoriously bad for one thumb typing so I thought if I was going to learn a new layout I may as well learn something that’s optimised for one thumb. As for whether it’s faster, I have nothing to really compare it to, I’m not anywhere near as fast as on my physical Dvorak keyboard (which I use significantly more) but I’ve never used anything else on touch screen.