We passed it around at the end of the night, in a six person circle. Three times. Everyone had the same BAC rating EVERY time. This is the most accurate system I have seen, and I have tried most of them. Probably 6 other units.
I've had this for a few weeks now and it's a lot of fun, great for parties. Though I wonder about it's accuracy. After my first vodka tonic I blew a .21, and it was only single. So I had some water and retested and blew a .04, which may have been more accurate. After my third (second 2 were doubles) with dinner I figured I needed to test after a glass of water, I blew .00. A friend did the same and showed nothing. But another friend who just put his beer down blew .11 I would recommend this company but probably not this product
I've found it easy to use. I've compared its results with one of the larger, more expensive models. It has varied a little bit, and I tend to believe the larger model was probably the more accurate. That being said, this keychain model is definitely close enough to be of great value for its purpose.