It works, as advertised. So why a three-star review?
Because after buying this, I discovered that there are FREE phone apps, like Guitar Tuna for one, which do a better job - they allow you to tune the guitar more precisely because the interface is more exact. The above-mentioned app shows you numerically how many hertz out you are on the string you are tuning. This tool just uses a dial interface that gets you really close, but not perfect.
In side by side trials I was able to tune the guitar exactly with the app; with this tool you can get very close but you will have to fine-tune by ear to get it just right. The interface just doesn't have the precision to get you perfectly in tune.
So I can't fairly complain about the device, which absolutely works as advertised, but I wouldn't buy it a second time.