To add not very much to the thread...
I loved the Psion Organisers, and OPL (Organiser Programming Language) in general. I had a break from 'computing' after the Spectrum/Oric 1 type machines had faded from view. I eventually had a requirement to store a database of over 5000 entries containing the common faults and remedies for the television repairs I'd amassed over (too) many years repairing TV/VCRs. I got a Psion CM (the two line 16x2 LCD type) and slowly started entering the list of faults. I soon upgraded to a Series 3 with it's better keyboard and superior display. From there I went on to have a 3a, 3c, Series 5, 3mx (I know, backwards, but the Series 5 had a poor contrast display that I never much enjoyed, the 3mx was simply superb) then on to a revo. The revo was a wonderful machine, if rather poorly made and saddled with a fitted battery that wouldn't retain it's charge so back to the 3mx again. I still have all of those machines, along with a Psion siena, Oregan Scientific Osaris, 5mx, infra red modem, Psi-Fax (as the name suggests, it allows sending a fax from a Series 3) MC600 (lovely laptop sized machine which came before the Series 3) . I also have a Sinclair Z88, Amstrad NC100 and the Amstrad PDA600 (though I can't see the PDA600 or Z88 for a minute...).
I wrote rather a lot of programs for them, mainly for maintaining the database of 5000+ TV faults. The database program I wrote (DataSel - for Data Selector) ended up with a feature I was really pleased with called 'Insta-Search'. If whilst viewing an entry you started to type something, after a short pause in typing, 'Insta-Search' would be initiated, searching the database for the next entry containing that phrase. If after typing a phrase Shift+Enter were pressed, the database would be searched in the reverse direction. My database program was written for almost all versions of the Psion (as I often swapped back and forth between them) and had a virtually identical feature set, whether running on the Series 3, siena or 5mx, something I was rather proud of at the time. I also wrote a number of programs that allowed the database to be viewed on a PC, originally in Visual Basic 3 (YUK! Hated it.) then in Borland's Delphi (YUM! Loved it.). I wrote some simple utilities that allowed me to calculate microwave power and free UHF channels for setting the modulators on videos so they wouldn't clash with incoming signals, these should be a minimum number of channels higher and lower than any incoming signals in order to minimize potential beat frequencies being generated.
A UK company called Sendo made a mobile 'phone called the Sendo X that had an in built programming language similar to OPL as it's OS was based on Symbian 60 which was the name Psion adopted for it's OS marketing shortly before they were swallowed up by Motorola. I actually had one. It was awful. Nokia made their communicator series of 'phones some of which (essentially) incorporated a Psion into the 'phone so they too could be programmed 'on the fly', but I'm not sure if they sold well as they didn't last that long and I never saw one.
Whilst it seems like a great idea to have the ability to program a device while out and about it seems as though it has limited appeal in the wider world. RFO Basic has the capability to run on Android 'phones, it can build graphical interfaces to make it look like a 'real' Android app, there is also an App builder that (I believe) wraps a version of the program up with the BASIC interpreter into one, self contained Application that looks like a genuine App to the end user. I downloaded it, wrote a few simple 'Hello World' type programs but never went any further. As I am someone who now writes code and designs electronic circuits for a living, I would have thought I would have been more interested in using an App like RFO Basic than most, the truth is, I simply couldn't find any reason to write anything for my 'phone that I wanted, or hadn't been done better already without the need for me to spend hours learning another programming language and syntax, then any other number of hours honing the results into something that worked any better than I could buy for a few pounds, saving myself many sleepless nights into the bargain.
It is still possible to get the Series 3a (3c?) emulator to run on modern PCs (or macOS) using DosBox, if I need to look something up that I had on my Series 3mx I do sometimes fire up my virtual Series 3a and have a look for it. I wrote an article for Psion Journal detailing how this worked, along with another article that was never printed due to the magazine going out of circulation.