No experience in an AR yet, but I did have a 22 Hornet H&R rebored and rechambered to 6.5G. Unfortunately my chamber was cut too deep and ran into problems after only 200rds were fired. I found it to be an excellent cartridge that is capable of much more than it's small size would indicate. I really want one in a bolt gun!
I've shot Melensdad's and it is a very capable round.
On the shooters side of the muzzle, there's only a little more thump than on a standard 5.56 ar.
On the other end of the muzzle you have probably the best long range ballistics that you're going to get from an AR-15.