There should be some unsorted notes related to pocket computers.
I have three types of these things: an Elektronika MK-85, a Casip FX-700P and a Elektronika MK-90. They are pretty different: the MK-85 is based on a copied design and human interface of the Casio FX-700p calculator. Internally, it is pretty different: the MK-85 is a 16bit computer compatible with the PDP-11. It hes an integrated BASIC interpreter (near fully compatible with the Casio's one, but with some enhancements related to graphics and to Russian language support). The main limitations of that small machine are a tiny display (just one line with 12 characters and with few additional symbols), no I/O and no easy way to write or install machine code programs. The main advantage of this machine is its size (it's basically a thin calkulator) and battery life (it measures in months).
The Casio FX-700p is obviously very similar to the MK-85. Most BASIC commands are the same but MK-85 had fixed some of the Casio's flaws (EXP functions problem) and so. A actually use the Casio more often for traveling as it is less precious than the MK-85.
The MK-90 is also internally compatible with PDP-11 but it has a large LCD with graphics support, large keyboard and an extension port. But it software support and ergonomy is much worse than the MK-85s one. There is a bare BASIC interpreter (but with support of of graphical commands) which is based on a PDP-11 BASIC from early 1970s. There are two battery-based storage modules for programs and data. These modules can also carry and autostart code for a special software.
I have two of them: an Atari portfolio (which is guite large and has
strange keyboard but has an excellent battery life) and a HP 200LX
(and a 95LX somewhere in storage). The HPs are nice but they have a
calculator-like keyboards. It's an excellent solution for
computations (for enter of numeric data and so) but not that nice
for fast typing (anyway it is much more comfortable than the Atari's
one). The main problem for me is sychronization of
calendar data with anything modern.
The 200LX is almost fully
PC-compatible so most stuff in the text mode or in the CGA graphics
should work. That's ideal for using of small tools (hand-crafted C
programs, Gnuplot, some tables converted to Lotus123-compatible
format) and for notes on the go. By the way, the integrated
calculator is also not so bad (it's a full implementation of some of
HP calculators so there is no need to carry a separate device). And
there is a switching between applications without data loss. It has
some limits (only few applications can be opened, only one DOS-mode
app and so) but this is a big advantage even over desktop DOS
computers. The LX machines use PCMCIA cards for data storage which
eases data exchange (PCMIA to CompactFlash adapters are available
but the size of cards is limited to about 32 MB).
I used the 200LX very actively for more than 2 years but now none of my ODS-based computers is in use (I sometimes test my software on the Portfolio to verify that it still works but that's all).
I never used pocket PSIONs for an actual work. I'm still use a PSION MC600 laptop but it's generally a MS-DOS based PC machine (with several specialities) with both all its advantages and disadvantages. It has an unbeatable battery life (up to half year on a modern batteries), though.
The Series 3 (3a/3c) PSIONS are competitors to LX series machines from the HP. They have better program switching, nicer GUI and keyboards more suitable for fast typing. There is an integrated OPL programmin language so one can build full GUI applications directly on the device. But the OPL is not exactly nice for me.
To be honest, none of my PSION PDAs is frequiently used.
There are many kinds of them - for example my Sharp Zaurus SL-C3200 is a small notebook-like Linux computer with SD and PCMIA slots and with keyboard that feels like the HP LX keyboards. It's less wide than the HPs but other dimensions are similar. The more modern OS and muc higher specs make possible to use of better (and colorfull) screen and of UNIX-like OS with GUI. The tradeoffs include not so good battery life (few weeks in suspend but under 10 hours in work) and need of non-standard battery. The apps are heavier so not all tasks are actually faster. But one can run GCC, LaTeX and office apps here. But the WWW browsing is rather impossible now (2015) as the available software does not support the most of modern stuff (HTML5, many parts of JavaScript and so).
There are also two sizes of Zauri with sliding keyboards but they are not so comfortable to use because of size and number of keys on these keyboards. By the way the SL-6000 is bigger and heavier than the laptop-shaped Zauri (but it has also a bit bigger screen and a massive battery).
An extremely high build quality (which was also reflected by an original price) have to be noticed.
I have found that use of a clamshell Zaurus is pretty comfortable in trains where is no table - any other device is harder to hold or to operate. Thus Zaurus is a preferred option for a short (up to 2 hours) train trips.
A tiny laptop-like computer with open source software and most of hardware. It feels like a (much) cheaper Zaurus. It supports charging over USB, ethernet over USB (that actually works) and more. It has some issues: its underpowered (CPU is fair but 32 MB of RAM are too little even for lightweight Linux distributions) and until the end of active development where was no reilable suspend mechanism. So it can survive about 8 hour on batteries and it have to be turned off when not used. It means some discofort to users. An another issue is an use of framebuffer - it's surprising how many applications has problems with that (it's near impossible to have a working setup of Octave + Gnuplot, for example). Unfortunately, only about 2000 machine was produced and now it's a dead device with no development. But a bigger issue is that is is a cheaply build - the hadware can wear-off relatively fast (screen hinge, supporting legs, keyboard).
I used to use the NanoNote on a daily basis: for calendar, to-do, reading of e-books and RSS news, for some kinds of lightweight computations, as a dictionary and so. But hardware of my devices became weared off and also I have got an Ubuntu phone which is able to replace the NanoNote for some tasks. It is also bigger so I decided to carry only one device in most cases. Thus the NanoNote was less or more abandoned (for my trips I usually use the Zaurus as a pocket computer).
The Palm Vx was my first handheld and I used it for a few years as a PDA, an e-book reader and a calculator. But I had problems with the build-in propietary battery so I sold it and got several Palm III devices (they use widely available AAA batteries). Actually I don't use any of them on a daily basis. The Palms were well-suported under the Linux/Unix systems (including a calendar and data synchronization, software installation, backups and other necessary functions).
I have a Kodak PalmPix camera for Palm III. It is quite old so it provides just 640x480 photos and it has fixed focus. The black and white display of the Palm makes the taking of photos a bit funny.
One of the very first Linux handhelds. Actually an excellent piece of engineering work: a small machine that was able to run real X11 environment (they used the FLTK toolkit but it was possible to run general X11 application, too). It was even possible to export the display to other machine and to do other unix-style things. There were limits: small and unexpandable data storage and limited memory. The box was able to play MP3 but there was a very tiny space to store them (just for a few songs). Also the synchronization protocol was bit problematic - I never managed to make PPP connection from IRIX (it works flawlesly from Linuxm though). The one of the biggest issues it it's battery life - I never got over one week in suspend which too bad in comparison with Palms from the same era.
The device itself probably didn't sold well and there was no successor.
Well, I have an BQ Aquaris E4.5 phone. It runs the Ubuntu Touch thing. I actually use it as a dumb phone (for call and SMSs, tahat is) but it has some supplementary possibilities that are usefull in some situations:
There are PIM applications but they are actually worse (in the terms of usability and configurability) that the ancient Palm applications. The only plus is an on-line synchronisation (but it is limited to the Google servers...).
Anyway, I use this thing on a daily basis. It's a pretty good phone, after all. Thing are continuously improving but there still some limits and problems. Some of more advanced Ubunt devices (the M10 tablet, for example) can run X11 applications and it makes them much more usefull but the E4.5 has so limited so Canonical(Ubuntu) developers decided to not add this possibility to this phone (it has no possibility of monitor output and it's internal space is soo small to accomodate another software subsystem).