I have one on my system and its very useful. I should mention that my system is DI only as I live in a soft water area.
The meter is good especially when changing resin manufacturers as you can see which one is the most cost effective to use, or if sticking to the same one you know pretty much to the liter when the tds is about to rise. I have been using it recently to test the two vessels in line theory. So it has many uses if your the inquisitive type but its useless if you don't care about facts and figures.