One thing I didn't know is you give up google voice is you subscribe to project fi. Very disappointed by this. I don't want to give away my real phone number to a million vendors.
For what it's worth, you can have two different Google accounts on your phone. One of them uses Fi, the other one "regular" Voice. That's what I ended up doing. And no, you can't make Voice forward to the Fi number, but that's less of a problem now with VoIP.