Update 9/9/11: The ZZoma now requires a prescription which I assume is the reason it is no longer for sale on Amazon. Search for the Zzoma web page for more details.
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I snore much worse on my back than on my side. The Zzoma keeps me from rolling onto my back, so I sleep much better using it.
It works far better than the old tennis ball trick, where you pin tennis balls to the back of your shirt to keep from sleeping on your back.
The Zzoma works best when you wear a well-fitting cotton shirt with it. Loose shirts allow it to move around a bit.
A couple of caveats: The elastic in the band that goes across the chest eventually lost most of it's stretch after about 6 months, allowing the device to move around a bit when I sleep. So now I just help keep it in place with a safety pin, pinning it to my shirt.
Also by the end of the night, my body must be a bit tired of sleeping on one side all night, and in my sleep I sometimes forceably reposition the ZZoma so I can sleep on my back. If I were awake I really wouldn't do this of course, but I can't control myself when I'm sleeping I guess!
Even so, overall the device works for me. I use it nightly and sleep much better now.