a few months ago I put together an AR, chambered in 6.8spc, using a Cavalry Arms lower. The reciever and butstock is a one piece polymer unit with an intergral buffer tube. So far I am impressed with the polymer reciever. It fits very tight to the upper reciver, cuts the weight of the rifle a bit and feels pretty good to handle. I have a low round count on this AR so I can't say from first hand experience how it will hold up over the long term but it should provide a decent service life. In short I like the polymer lower recievers.
Cool. I've only ever used original aluminum lowers, but I don't see why a well executed poly or CF lower wouldn't work. They don't really have much in the line of mechanical loads applied to them by design. One of these might be good to give you minimal weight. I wouldn't want to use one with too heavy of a barrel, though. I hate muzzle-heavy AR-15s, balance is an often overlooked virtue in a rifle.