For my needs I find the HDMI model a very useful companion for my future GH4; its a YAGH on steroids, its an external 4K recorder, it has an fullHD iPad-like video monitor, and more... all that in one single package, for me I found that convenient. 16 pounds of awesome, I know, a heavy price but its workable.
If I want light weight for run and gun gigs or get some quick shots just unplug it and you're good to go.
Now, price its not available for this model, if its more than $2k - $3k, I would not buy it... although thats the price for external recorders only, if you want XLR inputs thats another accessory...
Also, theres no info on whether it can be upgraded to add a sensor in the future.
I think it's to early to celebrate or complain, so will see.
PS: I honestly don't know WHY they used a 5D for the preview picture... makes no sense.