Below is literally the first result in a search for Bear Creek Arsenal Quality... the consensus seems to be "fair" and "you get what you pay for"... a wide range of opinions, though, to be sure.
Frankly, I don't know if that upper is amazing, total junk, or something in between. If YOU know, then you can assess whether that $200 upper is a better/worse value than a $400 (OBO) Sig.