My husband and I are delighted with our new carbon boiler.  At the moment we have it timed to come on for just a couple of hours early each morning.  This gives us enough hot water for normal household use every day.  If either of us wants a bath we just boost the boiler for an hour or so beforehand.  We find it works far better in our house than the previous combi boiler and we don't have the problem with poor water pressure upstairs and lukewarm water in the bath tap that we used to.  The carbon boiler provides hot water literally on tap anywhere in the house at any time of day.