I would totally concur on the GWACS, and in fact am considering getting one; it addresses many of the concerns with polymer lowers by not just duplicating an aluminum lower in poly. I recently watched the series that Ian and Karl of InRange just did on it, and am impressed.
I also am intrigued by the Kaiser Arms set, but have no personal experience. Among more conventionally styled polymer lowers, the Tennessee Arms receiver is well made, IMO, and I would not hesitate to suggest it where weight is a consideration, or for a rimfire build, but I would also advise anyone considering it that it shares the inherent weaknesses of poly receivers that emulate aluminum ones.
That said, ATI does not match the standards of any of these. Their products are like low quality injection molded toys with pot metal parts cast inside.
I've been using CAV-15 lowers since about 2003; it's amazing how much dead weight you can lose, without compromising reliability or busting the budget, simply by going with a GWACS/Cav Arms lower, and carefully selecting other reasonably priced, "off the shelf" components. I recently assembled a CAV-15 carbine that came in at 4.75 pounds (5.25 lbs with red dot sight, empty 20-rd mag, & sling). Holding something like that in your hands, you realize why our fathers & grandfathers loved the M1 carbine!
;^)