Problem is that Google Voice doesn't always use the same number all the time any more. So adding to F&F can be a problem.
I haven't really used it. have had it for almost a year i guess. Never check to see that they are changing numbers. Ya.. that would be a problem.
My issue with the SMS was, when i did use it. it was free on TXT messaging for me (both in / out)., but my wife didn't have GV and she was getting dinged for an "out of network" sms every time.