I bought one these things this summer.
And here's my reasoning:
-I'm in Japan, and it's here in front of me for $20.
-It's sexy
There you go.

lol (I'm an idiot, yeah??)
Anyway, it's as the pictures show. I run an air stone at night, so the indicator's usually a dark green in the morning, and at the "just right" yellow color by the time I come back from class. I'm not so good with sophisticated tests so this is a good tool for my level of intelligence I think.
Is it necessary? Not at all. XD
Is it sexy? Yes!! <33
Anyway, I have one, it's useful, and I seem to be getting the CO2 my plants need to them. That's all.