However, I've seen confirmation from at least two people that upgrading to the new Hangouts in Gmail breaks the incoming calling feature on the Obi using Google Voice. This is my largest concern as I've set up several friends and family members with an Obi using GV.
That is not the case. Enabling Hangouts has no effect on OBi for now. I just now tested it to be sure. The issue that IS annoying is that switching to Hangouts in Gmail removes the ability to make OUTbound calls from Gmail, using the Google Voice & Video chat browser plugin (now known as the Hangouts plugin, but it is the same thing). The good news is that you can simply click on your avatar in the lower left side of the screen, and "revert" back to the old chat, for now, until Google works this out. For more details, join us over on the Google Voice forum.