i think, in general, that those using tap water have many more problems with diatoms and problematic algae than those that use RO, yes?
a friend of mine here has a gorgeous 125g reef with tap water. he battles algae a little bit, but it's really not too big of a problem. who knows. it definitely can be done with tap, but evidence certainly seems to point to healthier fish and a less algae-prone environment. there is no doubt that RO water gives you more options for stocking fish...especially certain more sensitive, fragile species.
personally, i use tap. but if i had a 125g reef i sure wouldn't! i only have a 20g SW semi-reef with one damsel and various critters, so my options are very limited and my expenses so low that i don't see RO as worth it right now (since i can't get it at my lfs). i do battle an occasional diatom outburst, but everything in that tank has been healthy and lively since i set it up here in august. but, the bioload is very, very low, so that may have something to do with it, as well. when i upgrade that to a 55g reef eventually, i plan on buying the RO gear. hell, if that tank weren't in my backroom, i'd have it now, if only for the potential aesthetic improvement.