I've had an iPad of one stripe or another since the first iteration came out in 2010. I love this device, but one thing that was clear from the outset was that it would need a case. I've owned several, of various designs, and even briefly had one of my own devising(1).
The BookBook case for the iPad Air by Twelve South is, hands down, the best case I've ever owned.
Since 2010 I've been using an iPad in place of a laptop. Given that, I need a case to be protective, certainly, but I also need it to enhance the iPad's flexibility as a writing machine, and as a reference device when I am working at my iMac. It also needs to be comfortable in the hand when I am reading. The BookBook meets all of those requirements admirably.
Designed to look like a classic old leather book or journal, with a zippered closure, the spine is flexible enough to be flipped completely around backwards, and the case is rigid to allow it to function as a stand in both portrait and landscape positions. Flipped around backwards in portrait position is also a virtually perfect arrangement for holding while reading.
But probably the best part of the set-up is the typing stand. This is nothing complex - it's a rigid leather flap that folds down from the backside of the iPad holder in the case, and tilts the iPad up at about 20 degrees. The position is ideal for typing at a desk or table and, more importantly (from my perspective) for typing in a lap.
Undoubtedly, the choice of case is a very personal one, and this is reflected by the vast number of cases and other solutions offered for the iPad. Podcaster and Mac OS X reviewer John Siracusa has frequently referred to the design of iOS devices as working as the "robotic core" of a device that the owner then completes with his or her choice of case. If you are using your iPad as a work machine, it would be hard to find a solution that offers as much flexibility as the BookBook.
(1): some of the cases I've owned are shown below in that list of cases are