I just finished running flatwire to my side and rear surrounds. It worked out really well, but I had to make some runs across the ceiling so it required a bit of mudding + sanding to meet wife approval standards.
As others have said, it wasn't cheap, seems to be in the same ballpark as the taperwire, so I made some compromises by having the connection boxes behind a couch closer to the rear of the room and ran normal wires to that spot.
The reason I opted for flatwire was that they pretty good resources on installation and there were a variety of choices for connections.