We had a Bike Friday Tandem traveler XL, and all in all it was a nice bike. With an adult stoker, it was definitely more flexible than a typical full size tandem, which my stoker did not like.
It is not slower, in fact, they tend to be lighter than a number of tandems, and the smaller wheels offer less wind resistnece and spin up faster.
They are a bit twitchy (less gyroscopic effect) but you get used to it quickly.
Given the flexiblity of the frame, I'd be somewhat concerned if you're planning on loading a lot of weight on it. I'd definitely consider a trailer instead of panniers.
All in all, its a nice bike, with some compromises compared to a full sized tandem, but with the advantage of being packable.
Also if you're looking to buy a new tandem, why not a conventional tandem with S&S couplers?
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You could fall off a cliff and die.
You could get lost and die.
You could hit a tree and die.
OR YOU COULD STAY HOME AND FALL OFF THE COUCH AND DIE.