They do improve cheek weld somewhat. I got a knock off Chinese made attachment similar to this one at a gun show:
It fit VERY tight and actually took some of the wobble out of the stock, it was worth the money I gave for it (think it was about $8). They basically are attempting to turn the regular stock into a SOPMOD stock. All I use now on my ARs is the 2nd Gen LMT SOPMODS. Ever lower I have is wearing one now.
If it were me, I'd just save the money and use the ol M4 collapsible till I had saved enough to buy a Vltor Emod or SOPMOD. I got my last 2nd Gen Sopmod used for $75 shipped in like new condition in a different forum's classifieds. None of them how ever, do anything to help with a high mounted (carry handle or high ring) scope. For that I'd get a clip on or lace up riser. Hope that helps.