I plan on taking one montana cow but and a heads up decoy this year. I have a spot picked out that is fairly flat and open that holds some huge bulls and will need something for them to focus on so they don't hang up.
I like a decoy when I am hunting alone on the ground. When I am calling, the decoy can validate the idea that the bull has found the elk it heard. I think it puts them at ease a little. If I am out in front and off to the side of the decoy, some 20+ yards closer to the bull, I feel I have some advantage. I do not use any decoy when tree stand hunting. Elk coming into a water hole can be startled upon seeing the decoy and turn away. At the water hole, some elk are attracted to a decoy, some seem unconcerned about them, while others leave immediately on seeing one. Since I set up at a point where I expect the elk to come to, I do not want to risk turning them away. My decoys are a simple headless elk outline. I purchased a couple and have made some out of painted burlap cut outs.
I used one (Montana full cow elk ) for the first time last year with a client.
We spotted a group of 5 bulls milling around in the sage without much cover between us and them. They paid it absolutely no attention until we closed within a couple hundred yards. Once there, a couple of whinny cow calls and the mellow group of bulls suddenly switched, began sparring and tearing up the sage. We were able to close the gap to just under 100 yards while all this was going on and the client made a great shot on the biggest 6x6.
It was a blast - I am sold.
I too favor the Heads Up Decoy over the larger MT Decoys! I've slipped in on some pretty wiley critters with it even as close as 15 yards! It's easier to pack for most elk hunting situations. That new Slip Cover Decoy has its place as well over the Heads Up on blind/cold calling setups. I like the fact it will stand on its own & you can get behind it.
I have had deer and elk react both positively and negatively to decoys. I have seen them run off, frightened by my decoy, while I was waiting in a tree stand. I have observed them come up, put their nose up to the decoy and sniff at it, leave, get a drink, then return to sniff at the decoy again. I have observed elk react in a number of different ways to my decoys, but they do usually react when they see it.
I have to admit that I am surprised to see Elknut endorse a "Slip Cover Decoy", especially after he made a strong case last year that camo wasn't important to elk hunting success. From what I have read and observed about an elk's eyes, I doubt they even recognize what is printed on a significantly larger piece of cloth. I believe a plaid pattern or the picture of a bale of hay would do the same thing as a picture of an elk's head. The real advantage of a slip cover is that it is a very portable blind. I have never tried one, but if I am correct, a decent sized umbrella with masking tape strips, splotches of dried (no scent) latex paint or anything else that breaks up a solid color would do as good. I am not positive about my assessment of the slip cover, but I am very skeptical. Has any tree stand, ground blind hunter or hunter in a similar situation, set one up at a water hole and observed different elk's reaction to one while you remained totally quiet?