The Ball and Keg level indicators work, I have them in 4 or 5 kegs. As noted previously, they are probably most useful in a kegerator where you can view them from the front, rather than a keezer where you are looking down on them from the top. If there is enough space in between kegs in a keezer where you could get a good look, I suppose they could be useful there too.
They included a bunch of extra colored steel indicator balls, and I haven't lost any of them yet. It's a bit tricky to mate the indicator ball with the float inside the keg, but it's just a matter of practice. The only other trick is to position the keg, once the balls are installed, so that your gas and beer lines inside the kegerator don't touch the indicator ball when you open or close the fridge door, which will most surely knock the ball off the side of the keg.