FYI - several units are Ebay are dirt cheap. I've referred people on a budget to buy from there. However, you can talk to several vendors about those products. A club member brought one over and we tried to make it work and still couldn't resolve the issues. We completely tore it apart trying to find the source of the problem, and the lowest we could get TDS out of the membrane was 40.
Something about them appears to go wrong, sometimes immediately, sometimes a few months later. When talking with another vendor, he too agreed that he's constantly informed by people encountering this problem. They seem to work great as soon as they are installed, but six months later people begin to notice that the unit no longer functions properly, and repairing them or replacing their contents with new filters and a membrane is more expensive than the original unit.
Am I trying to scare you? No. You may even buy a great system at an unbelievable price, something we all hope to achieve on occasion. However, there is a distinct possibility you will not be happy long term. Just keep that in mind, read the feedback carefully and try to shop smart.
I don't believe people can provide feedback six months later, but odds are you can find discussion boards with comments on various products - Google is your friend.