I guess it really depends on what you're wanting it for... it's sorta a 'compromise' between all the other AR rounds, but excels at nothing.
For varmits, you can't beat the ultra fast 223
For long range shooting, the 6.5 grendel is tops
For coolness factor and better quiet knockdown power than the HK-MP5SD, the subsonic suppressed 300 blackout is the bomb.
And you get the advantage of performance nearly equal to the 6.8 SPC with the full power 300 blackout loads.
Another vote for the 300 blackout... since becoming a SAMMI cartridge last January, it's really taken off, now with new manufacturers jumping on board almost daily. Factory loads (both supersonic, as well as subsonic), factory rifles, uppers, complete rifles, as well as the full complement of reloading components and dies. Also, the bullet manufacturers are really jumping on board to develop lighter 30 cal bullets that will open up at lower velocities.