I'm a self-admitted keyboard snob. As such I'm very suspicious of most folding keyboards. After all, the keyboard has more moving parts than all other accessories you can add to the iPad put together. And moving parts are prone to failure. Once a single key fails to work on a keyboard, it's pretty much useless. For light typing use and for those who don't mind the cramped small form factor of most folding keyboards, they may work well. For those who are fast touch typists, those who use multiple keyboards (including full size ones), and those who type A LOT, I tend to be skeptical about the long term value of most folding keyboards.
Having said all that, the folding keyboard from Logitech may well be an exception. It's relatively expensive and heavy but it comes from a manufacturer that makes good, very durable keyboards and "unfolds" into a near full size keyboard. If I were in the market for a keyboard that can be combined with an iPad case, it would be the one I'd choose. As it is, I use the Apple Wireless Keyboard with the Origami Workstation keyboard case/stand. Requires a bag to hold both but the AWK is a very good and very durable keyboard so I'm happy. But the Logitech model looks to be an attractive alternative.