I purchased one about six months ago for cleanups when I do my three week water changes. They do a good job of filtering with the diatomaceous powder but they are a pita to use. They say you can simply back flush to clean but the powder cakes to the bottom of the jar and filter bag and needs to be disassembled to clean. They are also hard to prime and usually makes a mess of my floor by the time I'm through. The design is about fifty years behind the times, and the screws are not even stainless steel, its really not designed for salt water use. I would not depend on it to rid a tank of Ich, I know they say it can.
If you are willing to put up with it's shortfalls and keep it clean and dry to prevent rust it's great for spot cleanups.
I have the new larger one "XL" like new condition I'll sell.