Those actually work surprisingly well for what they are... I tested TDS on one eons ago for a customer and it was in the 1-2 range (I don't remember the starting TDS of the water).
They aren't a substitute for a good RO/DI unit but they do help.
If you're thinking about buying one though, I'd save that money plus a bit more and put it toward a real RO/DI unit.
I used to use one of those when I started out with my 20g. It worked great. Does go through filters quick, but since I only needed a couple of gallons at a time, it was perfect for me.