I just purchased my second Lyman 57GPR receiver sight and now have one of a .54 caliber GPR and one on a .50 caliber GPR. These sights move the distance at which I can accurately place a shot from 50 yards to 100+ yards. They are so easy to mount and adjust that I wouldn't consider any other sight for a GPR. It's even better than two of the other sights I've used and liked - the Thompson Center Tang Mouted Peep sight and the no longer available TC Vernier sight.
Pros: This sight doubled the range at which I can confidently shoot.
Cons: A bit bulky. But the great featues require a little bulk.
This review was written in the old system and had content requirements that are different than reviews written today.