Deer don't always spook from all smells not native to their environment. They smell things all the time that aren't native to their environment.
What spooks them the most is human odor.
The therma-cell does put off a small amount of scent, even though they say it doesn't. However, that scent doesn't bother the deer in any way.
Once you buy one, you'll wonder why you didn't get it sooner. And I second the poster that said to get the carry thingy for it. It has a strap on it that makes easier to carry and use. You have to lay it flat on it's back somewhere around you.
Once you see how effective it is, you'll take it with you anytime you are doing something outdoors where there might be mosquitos and flies....like a picnic or outdoor music festival, or pretty much anywhere.
My rock band played outside at night at Pickwick a couple of weeks ago, and I broke my Therma-Cell out and put it in the middle of the stage. We were mosquito, gnat and willowfly free for the entire night. All the guys in the band have bought their own Therma-cells since then, and none of them hunt.
It's just that effective.