I was extremely skeptical about them. I borrowed one from a buddy and used it before I dropped $500 on one. I had deer coming in down-wind of me and I was sold. Do they work 100% of the time on every single deer that comes in, no. But I've had 15 deer in front of me at under 20yds and you'll get one that gets a little alert and bolt out of there, and the others just keep on doing their thing. Then the dumb doe will come back and be fine. I've watched countless deer licking their nose and sniffing, but 9 out of 10 times they aren't bothered. They can tell something is a little off, but not enough to start stomping and throwing a shit fit.
I use mine in the tree-stand and in the blind. It's to the point to where I won't leave the house without it in my pack. I would recommend them to anyone who archery hunts. Besides the bow, its the 2nd most important tool I have for archery equipment.