Re: KNS Duplex FSP
The link in your post is directing me to a bad page on Midway's site, but I assume you are talking about the KNS hooded front sight posts that replace the standard A2 post with a duplex reticle/crosshair of sorts...
I have never installed one of these in my personal rifles, but I have shot other rifles with them installed, the last being a 16" M4 clone with rear BUIS and standard front sight base. For me...someone who is accustomed to shooting with just the plain old front post when shooting with irons...I didn't care for the duplex. It was more of a hindrance than a help. I had problems with the crosshair blurring on me at times (may be the design...may be my eyes), but that is a big problem when you can get the front sight (post or otherwise) in focus. Also, if you are looking to install one, I would consider a smaller aperture rear sight (NM or otherwise) in combination which would greatly improve the "sight picture" and may resolve some of the focusing issues I had.
As for the product...KNS builds good quality stuff. I didn't see anything on the ones I have used that would cause me to believe the part was inferior, poorly made, prone to breakage, etc. I think it just comes down to personal preference and individual experience with these front sight replacements as to whether they work for you or not.