I had a Ebay RO/DI unit (Devenob or something like that). It worked OK but not GREAT. The housing were built very good. The only thing that was cheap was the drain valve, the DI chamber (too small and ran horizontally, not vertically) and the membrane was really cheap.

You can get one and replace the bad items...but once you do your almost at the cost of a unit from Buckeye or AWI. I'd just go with a proven system for a few bucks more.