I don't use it. I know the idea behind it, and some of my meters are K-System capable, but I personally don't find them useful. The meters were introduced by Bob Katz in an attempt to standardize audio levels in the music industry in the same way the film industry has adopted a metering (Dolby) standard. Well, the Audio field is a different monster.

My most valuable meters are my ears to be frank, but sometimes (as busy people who work on Audio often times do) I am working while I'm tired, and that can skew your perception of loudness. I use the RNDigital Inspector package, which I have customized to give me alarms at the ranges I feel are needed while mixing and mastering.

I've set up two different kinds of meters/alarms. For mixing, I want an alert when my peaks reach -3dbFS and average levels get to around -15dbFS. For mastering, I want to know obviously when I've got an over (0dbFS) and average levels at -6dbFS. This is VERY loud, but hey, that's how hot levels get when mastering Hip Hop. I also keep an eye out on my AD/DA (HEDD 192) as that shows me when I hit an over and it stays that way until I reset the meter, which I find valuable.

These are used as a guide only, my ears make the final decision and there are times when I'm above and below my set alarm ranges.

Meters are useful, but in my opinion, not more important than your ears for making final calls.