Ballistically speaking, the 6.8 SPC falls midway between the 5.56 and the .308.
It makes an excellent mid-range (350 yards +/-) hunting round for medium sized game. Many states don't permit the .223/5.56 for hunting deer, for instance.
Although inexpensive plinking ammo isn't available, quality hunting rounds are no more expensive than that for any other caliber at about $1/round. Look for the price of 6.8 SPC ammo to come down as more manufacturers ramp up their production in 6.8.
XD-40 service, XD-9sc, member GeorgiaCarry.Org, National Rifle Association, Gun Owners of America, North American Hunting Club, Rocky Mountain Elk Foundation