I have the ultimate predator deer decoy. I haven't used it on elk, but I figure it's more effective at not getting shot at by other bow hunters compared to standing behind the cow elk decoy.
They work good but there are two things I think could be improved upon. The floppy ears and a better bow attachment system. It's a pain in the butt to hunt all day with it attached to the bow and it's not the quickest to take off and on with the velcro straps, so throwing it on at a moments notice isn't always an option.