A flashing light when you're low is not really good enough is it.....? when you're planning your flight, you need to take into consideration where you're going, the distance, airspeed, wind direction, anticipated groundspeed, etc.... will give you a rough idea on how much fuel you're going to use- providing you know your burn rate. Add around say a litre for reserve and you can get it spot on normally.
When i'm flying every so often i think about fuel level- but before i glance at my tank, i make an estimation of what i expect to see there- for example if i've taken off with 5 litres, and been flying for 40 mins then, i expect to see 2 litres in the tank.... I also know that i've got another 40 mins left for the mission, and 20 minutes reserve should it be necessary... This is a much better way of monitoring the fuel level.... instead of waiting for a LED to start flashing.... I certainly wouldn't waste money on this, unless it gave me a calibrated indication of what was actually in the tank, so that i could monitor burn rates.....