I bought CamRanger and loaded its associated app (from Apple AppStore) for iPad and it works well.
I wrote a review on Amazon:
"This device helps enormously with macro shots when camera is mounted on a tripod.
Composing and focusing are very convenient with a larger screen.
It is an excellent idea turned into a useful product.
Would be a tremendous add-on if the cost is less, nevertheless, I like it."
Best part of it when paired with my 5D3 (and also tested with my 5D2) and iPad, it gives live view, and with that I can choose which area to focus by touching the point of the image on the iPad screen, and the camera focuses at the correct area. To use something like this, a tripod mounted camera is very, very helpful as you don't have to hold the iPad and the camera at the same time.
Also, with a wireless connection, you can setup the camera on tripod to take photos of shy critters without being next to the camera so disturbing what you want to shoot.
And, you can shoot macros of flowers in a more dignified position by situating the camera low with a tripod and do live view.
True, the unit is $300, on the high side for an accessory, it is still fun to have.