I am currently working with 1-4 strings and 1-6 strings. I am very happy with both. I didn't get a 6 string key and never gotten a missing key response from the seller/manufacturer, but any 5 year old can pick a lock in 30 seconds and an old Pontiac key is perfect. fit. Both devices have been working for about 2 months without any problems. All connections are normal, the voltage is stable. The build and wiring quality seems to be as good or better than most other brands. I will buy another 6-string for my main house as soon as I find the right batteries for this system. I recommend this product.