In 2009 70% of poll respondents on this site said that they used RO water. However DI was not an option on that poll. It will be interesting to see if that has changed much.
RO/or no RO
http://www.canreef.com/vbulletin/sho...hlight=ro%2Fdi
I use RO/DI because it eliminated the source water variable. That way if I have something go wrong in my tank I can say: "well my lighting is still the same, my water is pure, the temperature is stable etc... so what have I done or added lately that could have caused this?" It makes problem solving much easier.