I haven't heard anything really bad against them, just the same 'ole from the earlier M16 replacement trials... they're good, but not good enough to justify the cost of replacing the M16 family.

The newest version of the XM8, as I've heard, has deleted the underbarrle grenade launcher bit. Granted, I've also heard of the army testing ~6.5mm versions of the M16, amoung other things, searching for a more efftective round then the 5.56mm SS109.

Who knows what they have planned next... the M16A4 suprised me when it was introduced (mostly by what a huge waste of money it is.)