Agreed. I believe the FAA regs don't strictly prohibit them, but discourages items that "alter the performance of any part of an airplane" - which obviously the knee defenders do. I also read on the zen website that it's not approved for use during take off or landing (duh! everthing is supposed to be stowed then anyway), and that's the only time I've ever heard an FA ask people to remove items from the seat pocket. Perhaps an arguement could be made as an item that would adversely affect passenger evacuation in the event of an emergency -especially if totally overstuffed- but IMO, it's no more obstructive than the various carry-on bags that are hanging out on the floors or the trays that are down.

Of course I'll remove it for my seatmate if/when they need to get past me. You have to close a laptop, put your tray table up and usually get out of your own seat when that happens too, so I don't see this as being any more inconvenient than that - in fact, with the design, it doesnt look to be any big deal at all. That said, if it interferes with my seatmates space (while seated) in any way, I won't use it period - and it'll either go back or on eBay.

Time & the first flight will tell... I know exactly the flight I'm going to test it on. If it passes muster with the usual FA on this flight, I should be golden. lol