Well, here's yet another opinion...

I've got a 5.56 RRA gov't model, and a RRA lower that I bought and built up the day Pelosi became speaker... I've been bouncing back and forth as to whether or not to go with another 5.56 upper, or to set it up with 6.8 or something else...
Long story short, I'm about to order a LWRC 6.8 piston upper. I've spoken to several operators who have used them in 'points East', shall we say, and can't break the damn things. Also, there are several posts at LWRC's forum, Lightfighter, and AR15, that - while they should always be taken with a grain of salt - seem to point to several gov't tests and/or contracts that are about to become public in early CY08, with regards to certain agencies and/or organizations adopting 6.8SPC. SSA ammo seems to be the preferred round, and there's plenty of examples of hunting success with this round, as well.
YMMV, and there's something for everyone out there... But as for me and my money, I can only afford to pick one 'bigger' (than .223) round to stock up on as my 'do everything' gun if the fit hits the shan, and 6.8 is it. If I find the links to some of the specific forum threads, I'll post links. (Been a few days.)
Take care, watch your 6, and vote guns.