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.
Otherwise, my experience so far has been identical to SteveL's. Leaving the old Chat client resident in Gmail appears to be the way to go for the time being, until they integrate GV and Hangouts the same as it worked with GChat in Gmail (or Obi can somehow use GV differently; I'm guessing not since it just uses the GChat calling functionality).