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?