I've used them. Mostly for shooting deer. One of my favorites.
I mostly use the 63grn SemiPoint but my Rem. Mod7 w/1/12" bbl shoots the GK as well as it shoots anything.
They are too expensive for plinking, but are 1/2 the cost of Nosler Partitions, and work just as well for their intended purpose of hunting light big game (deer and pigs, or such).
For "blasting" with an AR, stick with the FMJ. BUT if you have a 1/9" bbl, or tighter, and want a good hunting bullet, the Sierra is great.
However, my 1/14" .22-250 won't adequately stabilize the longish boat-tail, so I keep the 63gr FB on hand instead.