I bought what looks like the exact same one at Walmart this summer. It seemed to be OK until my camping trip last weekend. Due to the temperature the furnace was running a lot, and I was cooking inside as much as I could. The last night I checked the gauge and saw that it was down 1/2 way into the yellow. I was quite concerned, and decided to get an accurate measurement to see what had more propane; my tank on the trailer or the 5 pound tank I use for the BBQ.
I grabbed my portable scale
and measured my tank. As it turns out I still had over 15 pounds in the 20 pound tank.
Fortunately beforehand I had made a note of the full and tare weights of my tanks; and put that on the tank with a label from a label maker so I could read it better than the stamp on the side of the tank.
Yeah, it's a bit of a pain to have to disconnect to weigh the tank. But it is also accurate. The one thing if anybody is looking at this scale; you may be asking "yeah, but how do I read the numbers when holding 40 pounds?" Well the scale will automatically adjust until it comes up with a fixed weigh, and then beep at you. Then you set down the propane tank and the numbers stay on the display until you clear them.
I also bought this gauge
which should be somewhat accurate, but from the instructions it sounds like it would be easier to disconnect and weigh it. (Tank has to be recently used, water has to be a certain temperature, etc.)