SO--
They are exotic, high on the cool factor, shoot well, handle nicely.
Unfortunately, they are overpriced because of their relative rarity, hurt like hell if you try to shoot them left handed and mags tend to be expensive (I found all of my 30 rd. mags for $40 to $45, so you can find them cheap if you look).
At one point CDNN had Tan stock sets for cheap ($300ish), but I'm not sure if they have any more.
There are Stocks made that will take standard AR mags, but very few are in the US and no-one seems to want to make a contract run of them (probably not economcally viable).
I believe you are correct on the 16" barrel not being standard on the Green Rifles. Last 16" barrels I saw were about $800 without the foregrip. Might be able to get a 20" cheaper that that.
As an aside--the 20" AUG is pretty close in length to a 16" FNC or Galil with the stock folded, pretty impressive considering the AUG is ready to fire with that length and the FNC and Galil need to have the stock unfolded before they become much more than a heavy pistol.
Is the AUG worth the $3000+?
Depends on you--if you would rather have 3 or 4 post ban ARs or 2 Pre-ban ARs, or a .50 rifle and scope, probably not. But, if you already have a few of the above, the AUG makes a nice addition to a collection.