So how will this change the way voip devices work?
It doesn't change anything at this time. It's only a feature added to Google's own Hangouts mobile application. It neither adds nor removes any capabilities for third-party VoIP hardware or software that are currently using XMPP to interact with Google Voice. General comments have been made in the past with regard to "sunsetting" XMPP support, but at this time, that has not happened, and there is no publicly-announced plan to do so. Google is more focused on expanding the user base for Hangouts at the moment, then it is on shutting down any other functions.
Will the Obi device be able to use the Hangouts API?
That's the important question, and there has been no public information shared on this, nor do I expect there will be, until (or unless) that feature is released to the public.
Will this offer any new quality improvements or features?
The features added are those that have been added to the Google Hangouts app itself. What's significant, is that the various third party apps, such as Talkatone, GrooVeIP, Mo+, Phone+, GV Phone, etc, will become unnecessary. What's interesting, is that the Hangouts software contains licensed or open support for a wide variety of audio and video CODECs, which may offer better call quality over WiFi than the current, G.711-only support via Google Chat. It's irrelevant to OBi at this time, since XMPP support is still unchanged and available.
Google remains publicly committed to Google Voice and Hangouts, and I expect that near-future development will focus on adding and integrating more functionality to Hangouts. Beyond that, Google doesn't comment on future plans.