Correct me if I'm wrong Ash44 but I thought the biggest bearing on small domestic RO units was water pressure? Being that the higher the pressure the better the flow and less waste water.
I bought a unit from onlinebrewingsupplies a while ago that he no longer stocks. Definitely one of the best steps I've taken towards better beer, especially with regards to lagers. Each brew is consistent and there's no guessing about mineral content or chlorine levels. When I got it I put the RO water up for a taste test with my wife. I got her to try the RO water then tap water, and she said "you've added chlorine or something to this one, so what?" We drink water straight from our tap and to me tastes very clean. Whilst regular municipal tap water may not taste that bad, whatever you can taste in it is going to end up in the beer. Personally I'd rather the beer talk for itself.
I don't mix, blend or change anything between different beers. 100% RO water and salt additions as required.