Anything's pocketable with sufficiently large pockets ;-)
The Psion would fit well into a coat or jacket pocket, or cargo pants/shorts, and easily fit into a satchel or messenger bag.
The Psion S3 dimensions are 165 mm x 85 mm x 22 mm (6.50 in x 3.35 in x 0.87 in). Contrast the iPhone 15 at 147.6 mm x 71.6 mm x 7.8mm (5.81" x 2.82" x 0.31"), and the iPhone 15+ at 160.9 mm x 77.8 mm x 7.8 mm (6.33" x 3.06" x 0.31"). Other than thickness, these are pretty comparable.
What it offers most specifically, however, is a useful and usable set of capabilities, in both hardware and software: full keyboard, email, calendar, Web, word processing, spreadsheet, contacts.
I've known journalists who've used it as their mobile platform for gathering and filing news stories, which also means it passes the "professional use" test.
Somehow it's been 13 years and none of the available phone OSes have managed to be as usable or user-focused as WebOS was. Nor as easily hackable. The hardware was smooth like a river rock in your pocket, but still had an easily found physical slider switch to silence the device. It had wireless charging as a standard feature. And the software stack was familiar to any desktop Linux user with just a custom display layer on top. Complex multi-tasking was effortless on it (thanks in part to the gesture area and also the OS's cards interface). And it combined all contacts and messages regardless of communications channel into a unified interface. Oh, also, replaceable batteries!
Unfortunately HP played musical CEOs (three in less than a year) and one of them didn't see phones or PCs as businesses they should be in.
Former owner of Nokia E71, Nokia N900 (both user replaceable battery), Planet Computers Cosmo Communicator and Astro Slide (like old Psion keyboard): not really. The keys on this smartphone are very small, therefore annoying to use. The larger they are though, the bulkier.
You're also stuck in either portrait or landscapr mode. Whereas a touchscreen smartphone can reuse the touchscreen for say watch a movie or use different input (like say gamepad).
I believe the current innovation trend lies in those small foldables. Of course, small form factor and dual screen has disadvantages for battery, repairability, and ridiculously high price.
iPhone 4 was a great size and looked great, but the sharp edges would wear through your jeans in a way that the earlier iPhones did not. That's my only complaint.
