I've had one on my tank for 2 years now, and I really like it. It's just unfortunate that a tank valve isn't something I can easily travel with and slap on my rental tanks.
I've had two situations where it would've come in handy. Once I was in Mexico and a deck hand closed my tank valve just before I splashed. I was still doing the quarter-turn back like I'd been taught, and I'm sure he thought he was opening it. Luckily I noticed him twisting it and I was SURE I'd already opened it, so I did the three breaths test and saw the needle plummet.
Just recently I was in Hawaii with my own gear except for tanks and weights. I'm used to California diving where we do pretty much everything for ourselves, and in fact I'd set up my own gear that day on the boat. But then, while I was listening to the dive briefing, the crew went around and opened everyone's tank valves. The problem is that I have a minor issue with my second stage connection; sometimes when it's first pressurized, it leaks until you give it a little twist. I always lean in and listen for that hiss of air and fix it if needed, but the crew didn't know to do that, and I didn't know they'd opened my tank valve. So it was just sitting there, slowly bleeding, and had lost 500 psi by the time I noticed. Had the valve been green, I would've seen it right away.
Of course, both of those could also have been avoided by other people not touching my stuff. But then, the indicator valve is also a safeguard against my own stupidity, something that will surely rear its head as soon as I lose my temper over people touching my stuff.
