I'm happy to forward my experiences :-) I've had my 6400C for a while now and have put hundreds of rounds through it. By far, it is the best AR rifle I've owned. I had a post-ban Colt 6601, but the 6400C is a much better shoot.
The main thing I tell people is that it hardly has any kick at all. In general, ARs don't have as much kick as say an AK does, but the 6400C has perceptibly less kick than other ARs. This makes it very easy to put muliple rapid-fire shots on target. It may be a result of the muzzle brake. Many here have commented on it's looks, but I'm here to say at least it's effective.
I have mine 6400C attached to an EoTech 552 and the combination is VERY accurate. I have it dialed in and I can fill a 9" pie plate easily while shooting off-hand at 100 meters.
It's strage, but I feel like the 6400C is more of an extention of my arm rather than something that is attached to it. I haven't had any problems with it - no misfires, misfeeds, or breakages of any kind.