I just returned from my first field test with these. http://www.toysrus.com/product/index.js ... Id=3504424.
They work just like I wanted them to. how many times have you had a deer ghost in from behind you or thought that the slight noise was that pesky tree rat again. I got tired of not being able to see back there without turning and spooking a deer. I came across these from someone on the Jeepforum.com for using them when they take their hard doors off. You still have to have one side mirror in most states. They have a nice large clip that clips to small branches or my two section bow hanger. Once positioned the slight concave mirror gives me the view that I want. Its clear and not distorted. It has two pivot points to get it just right. For the cost (2 per pack) you can't find anything better, I've looked.
JR

Never seen anything like that before. Do they spook the deer at all?

They don't spook the deer that I could tell. I guess if the sun was behind you and the deer got just the right spot they would see the reflection of the sun. But I've seen reflections of sun light off wet leaves, bark, etc. that would be that bright. Your movement to turn and look behind you would spook them more. The mirrors are only about 1.5"x3", my range finder is bigger that that. I have a few trees along breaks that I hunt out of that they wouldn't be needed because of the under brush behind me, couldn't get an arrow through it. But other trees that are out into old growth with very little brush on the ground and deer can come up from any direction, that's where they are useful. I used to hunt out of ground blinds that these would of helped. Open a side window just enough to allow the mirror to clip on the side of the blind and to see that area to the side and behind without opening the blind up and being silhouetted from the main window.
During my field test I was able to repeatedly check the area behind me with a slight tilt of my head. This kept me from turning to look because I had several tree rats and birds making a butt load of noise in the leaves. If they help me from spooking one deer than the $6.00 price was well spent. If I think about it next time I'm in the tree with it I'll try and get a picture of how I have it set up.
JR

Sportsman's Guide also has a "deerview mirror" that looks promising for the purpose. It is $19.95, has a steel clamp on one end with a steel flexible conduit with some length that connects to a wide rearview-type mirror. I've always been interested in it, but haven't squeezed the trigger. Getting to the point where I need an assistant to carry the gadgets.

The weight and cost were the reasons I didn't go for the "Deer View". My pack weighs enough as it is.