I've had several over the years, and saw a couple at the Montamisé fair last Sunday. They're gorgeous and very heavy little things -- bodies machined from solid stainless steel, as far as I recall -- but I never could get on with the small square negative. Besides, nowadays I buy cameras to use or sometimes to write about, and a Robot (even the Royal 36 I saw at Montamisé) won't give me better pictures than my Leicas.
Edit in response to Rich and feenej: No, I disagree that they're too expensive. They're like Leicas. That's what they cost, because they are beautifully made and last forever. If they're not worth that sort of money to you, that's another matter, but if you've actually used one you are unlikely to compare it to any Instamatic 126 ever made.
I've had a Tenax too, but I find Robots nicer and easier to use, with a choice of good lenses.
Cheers,
Roger