It all really depends on the application you intend to use a Bluetooth keyboard for, and how many times you need to pack/unpack it. A fold-up that is reliable but somewhat vulnerable to moisture may be the best option for taking notes in a college class that you don't have books for, while a water-resistant roll-up keyboard may be needed if you're using the Pandora to organize amateur storm chasing, and the basic keymat may be the best choice if you're using it to pass the time or... <GASP>... actually get work done while waiting at the mechanic for your car to be done.
Most UI devices are individual-choice things. Mechanical reliability is generally a universal, but everything else about any UI device depends greatly on the use, the expectations of the user, their posture, or the size of their hands.