They are as accurate as the low fuel light.
I have one and like it because I don't have to keep track of my odo reading which isn't always an accurate gauge depending on traffic conditions I'm driving in.
If you've fixed the level float, then it is more accurate, mine has been fixed and the light comes on with the gauge reads E, at that point, I've got 30 city miles left or 50 highway miles until I'm empty.
The connect to an open connector under the tank, there's one for the gauge, one for the clock. They are easy to fit, most people mount them so the face is flush with the rest of the gauges, I've set mine lower on the bars so they are about 1.5" behind the primary gauges.
You have to be careful if you do this and be sure to leave enough room for the main gauges to flip up without hitting them so you can still lift the tank without pulling all your gauges off.
I also needed them set this low to fit my hippo hands on in the winter.