So far I haven't been able to test it, but first observation is that it won't fit on my iPhone 4S when I have the bumper on - which is always. The official Apple bumper is not easy to remove or put back on so I wonder if there is some form of extension available that can help you join these two together...
On the other hand it fits nicely on my trusty old iPad (gen one) since that one has a bumper that's open in the bottom.
I'll probably write up a review when I get the chance to test it properly - since it's not much fun using my own credit/debet card to pay money to myself, but the iZettle can be compared to Square which you can find in the USA. The main difference between these two is that Suare uses the magnet strip in the card and the iZettle uses the chip.
UPDATE: I bought the Apple Digital AV Adapter yesterday - since I needed it anyway. I was hoping it would fit inside the bumper, but no luck. Seems it's half a millimeter too big. No use grinding it down using the Dremel either since the casing around the plug is too thin.