The cartridge is a lot like a .223/5.56 necked up to .270.
Bullets are heavier than .223, with a bit of velocity loss. Still, it hits harder than .223, especially if you're comparing milspec ammo in both calibers.
With softpoint ammo, it makes a better big game (i.e., deer) round than .223.
Ammo is much, MUCH less common than .223. Ask for 6.8 SPC ammo in many sporting goods stores, and you'll get a blank look. (Uh . . . I guess with today's hiring practices, you can get the same blank look if you ask for .22 rimfire, but I think you know what I'm getting at.)
Bottom line, from a ballistic standpoint, there's nothing wrong with the round; but due to availability issues, I have reservations about it's practicality. If you want to hunt/plink with an AR and will provide your own ammo 'way in advance of your needing it, fine.
But for a "go-to" rifle in case of trouble (civil unrest, zombie swarms, whatever) I personally see little advantage over .223; not being in the military, I'm not bound by the Hague Accords in ammo choice, so I'm confident that the .223 ammo I'd use would work just fine. And .223/5.56 ammo is VERY common. So I don't see a 6.8 SPC, 6.5 Grendel, or anything of the sort showing up in my gun safe anytime soon.