Well it's been a month now since I have posted this poll and about 2 months since I started using RO/DI myself. It sure is very nice to be able to control water parameters to your liking and experiment with different levels in different tanks, it also makes killing snails very easy by lowering your GH and KH to the point where their shells become weak and brittle. But most of all I have noticed in general that my tanks are cleaner, spot algae is less than what it used to be so that is nice. The biggest advantage is probably to the fish in my case as my tap water has heavy metals in it which do a great job at killing my fish if I use a water conditioner that does not treat for heavy metals, I found that out the hard way. Now I don't use any conditioner at all.
I bought my 100gpd filter from aquasafecanada (ebay seller) for around $150 with the 3.2 gallon container (a must IMO), shipping and a digital TDS meter. That's quite cheap and based on what I hear about replacing filters, the only one that really costs a lot is the RO membrane and DI filter, both of these have a nice long life span and in 4-5 years I'm not sure if I'd spend $80 on new filter elements or just buy a new unit from them again, price difference is not a lot if you think about it (excluding the TDS meter and 3.2 gallon container). Plus I now have very good tasting water and ice cubes
Considering what I spend on my tanks each year it's not very difficult to justify $150 on pure water.