For the GH4 at least I've found this is useful in a few circumstances, but it isn't a substitute for a monitor.
It's useful to control the camera when it is inaccessible (eg on a gimbal rig so you can adjust exposure etc without physically poking the camera). I use an iPad on a bracket for gimbal rig monitoring when I can't see the camera screen, and it is much better than nothing. Touch screen to autofocus is particularly useful...
... but only before you start to roll. As a monitoring solution the preview image is too low resolution and has too much delay and drop-out to use for assessing focus. You certainly cannot reliably use it to preview or control focus pulls.
It gives a useful "where is the camera pointing now" or more precisely "where was the camera pointing a slightly random short while ago". But in all honesty it's no substitute for a decent monitor with its own waveform and peaking tools.
What I'd really like is something with the functions of a decent monitor which allows touch-screen control of the camera and full-HD preview image. Anyone want to put together a cabled solution for an iPad/iPhone? The screens on these devices are up to the task, it just needs the bandwidth of a wired connection rather than an ad hoc wifi network.
Cheers, Hywel