Ok, it's been a while and whilst I've been playing MGT this book is another one that irks me.
Nothing wrong with the writing per se, I've realised that what is bothering me about this (and other MGT books) is an editing problem. Mongoose appear to be trying to bulk out books instead of either cutting the page count or chroming up the product.
My main (and minor problem) with Psion is FOUR PIGGIN' PAGES OF PHOBIAS. Seriously, other books have phobias, GURPS, Call of Cthulhu, all have fairly concise lists of phobias without using up so much space. Fair enough, I can't think of another game publication with such a comprehensive list but do we really need it?
My other editorial irk which pervades most of MGT is the great swathes of EMPTY SPACE. Half page, quarter page, eighth page. Psion is full of them, I even found a page with a quarter page of text, a quarter page of art and half a page of empty space. When I'm paying £15 to £20 for a book I expect it to be packed. Hell, fill it up with character vignettes, drabbles, art, random tables just don't expect me to pay money for empty space again.
Now after that rant can I say that I'm finding Psion to be quite useful but again just missing the mark for me. I'll give it 3½ stars out of 5. (4 or 4½ if it was better layed out and edited)