5th SFG(A) trialled the 6.8mm in combat, the results were not sufficient to convince SOCOM (much less the Big Army, USMC, etc.) to adopt the round.
Any use of it at this point is being supported out of unit op funds, if at all.
Supposedly, the marines are now giving the round a look, however.
That said, I'm with RockyMtnTactical -- while it is a good round, I just don't see the military adopting it, for better or for worse. Partly because of the cost of replacement and partly because 5.56mm is doing the job quite well. The 6.8mm brings a different set of strengths and weaknesses to the table, in my opinion, rather than being an across the board superior round to 5.56x45.