This 6-stage RO/DI is similiar to the unit we use here (ours only has one DI chamber), but it has served us very well, way over 3000 gallon pulled from it to date. I've only had to change the DI resin once ($18 for approximately three refills). Best $100 we've spent yet. Do yourself a favor and grab a TDS meter at the same time. The whole thing will pay for itself in months compared to buying water.

http://cgi.ebay.com/AQUARIUM-2-110GP...QQcmdZViewItem

To replace all the filter elements and RO membrane is around $50 with shipping, I'm figuring I'll just replace the whole unit for the newer model when that day comes.