I used this to build a $1,200 7lb gun (unloaded, and with irons), and it's working fantastic. The rail is loooooong which let's you get great control of the muzzle, even in rapid fire. That length also extends the sight radius way beyond a normal carbine, and into rifle territory which makes accuracy with irons amazing without settling for a longer gun.
I have an Aimpoint Micro on it right now (light and great for competition), but I got to play with a ACOG the other day, and it was punching sub-MOA at 300 yards.
Today I weighed the just upper receiver group with one of the rail sections, my Magpul vertical grip, and irons (no BCG or charging handle); it clocked in at 1737 grams or about 3.83 lbs.
For the price, this is probably the best choice out there for a lot of people.