TDS Meter?

I was looking around for a TDS meter. I saw this online. Anyone know anything about this unit. I don't know much about them. Cheap price so not holding my breath that its any good. Any info would be a help.

TDS Meter for Aquariums, Hydroponics, or RO Drinking Water PPM Tester

i've never heard of it but for that kind of money ($20~) it seems worth it to me. Don't forget to buy something to calibrate it with......TDS calibration solution.