I own a GH3 and like it for its imagery and ease of use. I rarely use autofocus but I have noticed with autofocus that if the light is even slightly low, it can suddenly without warning go wildly out of focus. I don't mean slightly soft, I mean totally out of focus so that the image is a complete blur. Has anyone observed this on the 70d or is autofocus on it more reliable?

I suppose how frequently it happens matters. I have seen it once or twice on our 70D when it's dark.... if I up the exposure to brighten it up I haven't seen it. So, for me it would be a problem to use on the dance floor, or maybe speeches at a wedding reception, when I allow it to look darker.

I am exploring the 70d focus limitations and am developing techniques to work with them.
I find that knowing when to use each of the three different autofocus modes is important, as they will hold or lose focus in different ways with different subjects (especially moving subjcts) and under different lighting conditions. I think it is really useful to suspend the autofocus at the right time also, so the camera doesn't get confused. To that, you can either tap the screen or press the flash enable button to turn it off, or if you program the zoom button, you can hold it down to suspend the autofocus and release it to reactivate.
I also drag my finger on the touchscreen to follow subjects when the camera is confused. You are also able to program how the tracking focus gives priority to either 'locking on' or being more 'responsive', depending if the subject is moving a lot, or is fairly stationary, but you have to delve into the menus to do so.
I find that not being able to use the touchscreen and an external monitor at the same time is unfortunate, and I hope it is fixable in a firmware update, as well as the ridiculous app that cannot control video, or work on an ipad. Also, if they add zebras and a histogram that can be used while recording, as well as being able to control audio levels, this camera would be a video monster.
But likely we'll have to wait (a year?) for Magic Lantern to add features that should easily be included by Canon. How annoying! But anyway, this is a pretty great camera as is!

Finally found a GREAT side by side comparison video. It starts with the 7D, 70D, 5d mark ii, and 5d mark iii all filming side by side so you can see noise, moire, and aliasing for yourself.

Then he shows how the live autofocus and such works - skip to about 10 minute mark for that.

Do you have a link?

Woops. Sorry

Canon 70D Camera Review (vs 7D, 5D Mark III, 5D Mark II): Video Recording, Dual Pixel CMOS AF - YouTube

Here is a Tips of Our Fingers - Behind the Scenes from HUMBLE TV of a short film they Done with the 70D. They show how they use the autofocus If you click the Humble TV you can look up the full short they did.

Tips of Our Fingers - Behind the Scenes on Vimeo