I have an Obi, and it is awesome. Cost me $30, and no monthly fees when used in conjunction with a Google Voice account. However, Google announced that it is getting rid of its Google Voice service soon. I got an email from Obi saying that their system wouldn't work when that happens unless we get another SIP provider. Have you got something in mind for that eventuality?
I hold out hope that Google won't get rid of Google Voice. It has been a great service.