Around where I live, seems like the 6.8 SPC is more popular round. The big deal is that Alexander Arms is the only manufacturer I've heard of making the 6.5 Grendel upper, since they own the round. All the major manufacturers are doing 6.8 uppers though: LMT, DPMS, Bushmaster, Barrett, Rock River, etc.
Its on my planned list of stuff to get, a 6.5mm grendel hunter upper. the way I figure its got enough punch for deer hunting and not to mention where I live in NY its bear country, though I have yet to see one.