The first unpleasant surprise has to do with porting over Google Voice numbers. If you already have a Google Voice number, Project Fi will give you two options, one is to use your Google Voice number with Fi, and the other is to get new number which will result in you actually losing your Google Voice number. This is a rather odd decision by Google although perhaps there are some technical issues associated with being able to get a new number while retaining your Voice number. On the other hand, Google has always prohibited users from having two Google Voice numbers forward to the same phone, so this may be an extension of that policy.
If you do choose to port your Google Voice number you'll lose Google Voice legacy (not Hangouts) app functionality and you'll no longer be able to send or receive calls through those apps. Much of Google Voices functionality such as call forwarding, blocking, and voicemail transcriptions will move over to Fi. This also means you'll lose support for devices like Obihai which allow you to have a home phone through VoIP.
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So what do you think about the changes Google is making in terms of Google Voice and Project Fi? Will they hold you back from switching over to the new MVNO service?