If you have a desk, or at least some half-way comfortable place to sit down, then these might be an option.
But when I'm in such a situation I would rather use my laptop, which I'd have with me most likely, if I have a bag that's big enough for the bt-keyboard in your link.

Where my N900 really shines is in crowded trains, where it's even hard to find a place to stand comfortably. If you ride by train, then you surely know this situation, as it seems to be pretty similar in Poland and Germany:
You have 30cm to move in any direction, can hardly hold on to any grip nearby and have to "surf" the bumps in the track that hit unexpectedly every few seconds.
In these situations I need a device that I can hold in one hand in any orientation, because I'm not a professional "surfer", and I need to be able to get back to work quickly without re-adjusting to much stuff, after the occasional stumble.

The N900's fixed-slider-design is perfect in this situation. So would be a clamshell device with a stable hinge. My 9" netbook is a bit too bulky in the end, but I think a Pyra (or a GPD for that matter) would do fine.
Wobbly hinges, like smartphone+bt-keyboard in a common envelope are not that great because by the time you re-arranged it after a bump, the next bump will probably hit you and you won't have any time for working. Anything that's comprised of two totally seperate devices isn't even worth thinking about in this situation.