I have used the old techstar (not the organiser type) for about ten years. Yes I did use it for FAA exams. Much friendlier than the whizz wheel!
I'm not certain but I think the FAA only disapprove of programmable calculators.
Anyway, the bottom line is that nowadays I would'nt buy one. My Psion Revo is indipensible and you can get free downloads of aviation stuff that can do all that a Techstar can and much more.
Plus the other features like organiser and office type prorams are really good. You get a real keyboard too!