Biggest uses will be transportation, construction
Agree with Paul987, mobile is an area that might gain some minor advantage, but other areas are far greater. I see huge applications in areas like transportation: cars that are much lighter, yet much stronger and safer, electric vehicles that can run much further on a battery, aircraft with greater fuel economy. Hey, even stronger but lighter bicycles if the stuff isn't too expensive.
Space exploration, where every gram of weight put into orbit takes about $20 in fuel (that's $20K per kilogram), is a prime area: The potentially higher cost of working with a material like graphene might be recovered entirely from fuel savings. We're talking about satellites that cost $200 million to build so they're not going to quibble about the high cost of a graphene part; cutting 100 kilograms of weight out of the satellite could cut launch costs by $2 million or more, particularly for high-orbit launches.
Imagine stronger bridges, stronger office buildings.