I've been using GV (since GC days actually) with Gizmo as a landline at home , mostly as a number to give businesses that want my phone number (think cable/gas/credit card companies) and it's been swell. I've got an ATA to handle the SIP so I even got a traditional cordless handset from my landline days hooked up to it.
I want to get friends set up with something similar and want to play around more with GV with a cellphone + SIP as well but since folding Gizmo in, Google has closed off signups (you can find gizmo accounts being on auctioned off on ebay!), it's been a waiting game. I've tried other SIP providers but they never work as well (more lag, usually).
Besides, I figure they are working on something pretty awesome. I mean, if I can already rig up a free landline as it was, I can only imagine what actual integration with Gizmo technology can lead to. Death of the phone companies? One can dream right?
So hurry up Google! I want to see what you've been working on with this stuff for the past 6 months!