"It's a possibility users would be left in limbo, like Google Glass," Entner said. He recalled that Google launched Goog-411, a speech-recognition-based business directory search, in 2007, only to abandon it in 2010. Google later admitted it used the service to gather a large database of voice sounds to be able to improve its speech recognition engine.
"I don't know what Google expects to get from a beta user base except more data on how well the service works and how well the Sprint and T-Mobile handovers go," Menezes added.
Loser: Small carriers
While Project Fi is sure to stir up interest, Google's success is bound to hurt smaller carriers, including those that operate over both Wi-Fi and cellular but don't have the name recognition, cash and mammoth size of Google.
"Overall, I think Project Fi is a positive and any new innovation is positive for the market," Entner said. "It shows how open the market is that you have something like a Google offer a differentiated wireless service."
But, he added, "the small guys will suffer the most — the Republics and the Tings. Google is like WalMart coming to town. It's not Kroger or Stop & Shop that suffers, it's the mom and pop stores that will die."
According to its website, Republic Wireless offers phones that work on both Wi-Fi and cellular, and the carrier commits to offering phones that are optimized to use Wi-Fi.
Ting began wireless service in 2012, and in March it became the first North American carrier to offer shared usage over both GSM and CDMA. It also launched gigabit fiber services in Charlottesville, Va., in April.
A final concern is customer service. Google has experience at customer service with its Google Fiber launches in the Kansas City and Austin areas, but customer service could be Project Fi's biggest hurdle.
"Customer service is one of the most maligned parts of wireless service and it's extremely difficult," Entner said. "To see how Google solves the customer service problem will be very, very interesting. I can just imagine a service call to Project Fi where the customer starts asking about all the things Google already does, like how to use Google Search or Hangouts."
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