Such a beautifully-written article, but missing a few details. No mention of the PsiXpda? It had the Psion form factor and ran Windows XP. It was well reviewed but poorly advertised, and the company went bust in 2011. Old units are still available.
I started with a Psion 3a, then 3c and 3mx. I wrote dissertations on them, programmed my own apps, and ran courses for university staff. (Imho, Steve Litchfield's "Fairway" golf simulator was a masterpiece of efficient programming.) The 5mx was physically well designed, but I found it quite buggy and inflexible in some ways. The Series 7 netbook was nifty, but soon eclipsed by standard laptops.
When Psion was discontinued, I switched to a Palm Tungsten C and wrote a PhD on it. Discontinued in 2004. Then I used the Fujitsu Stylistic series of 2000/XP slates: which were fabulous until they switched to Android. So now I'm trying to get by with a Samsung Galaxy S3 and an MS Surface Pro, which are both quite capable but awkward to use if you're trying to "produce rather than consume". Every device now seems to involve a compromise. I yearn for an instant-on handheld machine with a usable physical keyboard ... any ideas?