Can't add much to an already great discussion, but I have shot my buddies LWRC M6A2 in 6.8 SPC and it is very very accurate. Slightly more felt recoil then 5.56 as expected, but not much. He wanted more "boom" then a 5.56, but after getting it he doesn't shoot it much at all due to high ammo cost. I like to pull the trigger a lot, so I am happy that I stuck with my Sig 556 even though it's not nearly as accurate as his LWRC.

Here is a good 5.56 vs 6.8 discussion (once you weed through the typical forum drama) over on the LWRC forum as well:
http://forum.lwrci.com/forum/viewtopic.php?t=5310

And here are some interesting ballistics get test photos for 6.8 SPC:
http://www.68forums.com/forums/showthread.php?t=1778

Edit: Here is another interesting report linked in that LWRC thread:
http://www.dtic.mil/ndia/2008Intl/Roberts.pdf