In theory I actually like the idea of having my SMS / Phone calls / emails / hangouts merged pretty tightly, but there are a lot of subtle ways that they could really screw up the user experience while making that happen so I'm still nervous about it.
you have to keep switching accounts (close conversation, press hamburger icon, click account icon) everytime! got a SMS? switch to the confusing SMS account. Got a SMS on your google voice account? switch to the account (gets better if you have two!).
Dial via google voice? had your SMS account selected last? you will only get a "can't dial" error and the app closes.
it is the worst wtf fest of all google screw-ups in usability ever.
EDIT: I see now, I think my experience is different from others in that I only use my Google Voice number for everything. Doing SMS to/from your actual phone number inside of hangouts is definitely a disaster. If that were my use case I'd be using the Messenger app. If you're exclusively using your Google Voice number, though, the integration is pretty straightforward.
is there a way to access gmail once you've switched to inbox? I would love to be able to edit my filters again...
(Unless you turned on the auto-redirect to inbox, in which case you need to turn that off: https://support.google.com/inbox/answer/6230358?hl=en)