The number one difference is forged vs milled. Companies putting out the majority of AR lowers are forging. Milled are less available and cost a bit more. I have a DPMS, Colt, RRA, and Sun Devil. There IS a difference, and the milled are much nicer IMO.

Biggest deciding factor is what you are going to be using the gun for. Just want one and plinking, go with a reputable forged lower. You gonna shoot competitions and dump thousands of rounds through it, spend the extra Benny and get a milled receiver. Going to try to push a bullet past 600yds regularly with it, the milled might be the way to go too.

Biggest difference I've seen is the fit between the upper and lower. The milled seem to be really snug. All the milled receivers I've seen just plain looked better too; crisper lines.

Are they so good that you just have to have one? If you have the money and means, sure why not. Is it going to make or break a gun build, not even close.