Many here will say that the 6.8 makes an AR what it should have been. I guess I am one of them. It is capable of great accuracy from a familiar platform, with more energy on target, and very good (and growing) industry support.
It was targeted for the .Mil in 2004 but a screwed up chamber made it look different from what we use today. Today's 6.8 is another animal with more power and the ability to run well from short barrels. Most of us still have a 5.56 for blasting or 3 gun but when you need some more power/reach, the 6.8 wins hands down. It along with the 6.5G/.264 offer the most out of a AR15.
Some states, no deer hunting, most states 6.8 is OK. The 6.8 also has a handful of premium hunting rounds. Many Pig hunters here have shared and compared 5.56 to 6.8 on tough hogs, 6.8 does a better job on the tough pigs. May pigs are slain with 12 inch 6.8's to 300M.
In a Personal defense mode(patrol, HD, Military), the 6.8 can deny cover where a 5.56 round might not penetrate. It's an accurate cartridge, and will kick a two legged predator a## as well. For LEO's who work around cars a LOT, the 6.8 has barrier blind rounds that ROCK, I think a better tool vs the 5.56.
Only thing the 5.56 has over the 6.8 is ammo cost is lower. But 5.56 dedicated real hunting ammo is about the same as the 6.8 in cost. For Models...check out the vendors here and depends what you want to spend. You can get a great shooting 6.8 for cheaper one can think. Some buy out right but I followed advice. I did a "Build" that was a complete upper (cheaper than building from scratch for me) and a complete lower. I got an ARP for greater range and performance to compliment my Mini 6.8 I use for grunt stuff.