We bought an RO/DI when we got the tank, because I didn't want to perpetually buy and transport water, and we have enjoyed an unexpected benefit:
that water tastes much better than anything you could ever buy! My wife and I drink probably triple the water that we ever did before (more in line with how much water a human -should- be consuming in a day).
I did notice the amount they say to use on the mix instructions doesn't seem right (mine mixes low), but it is consistent - so I know how much I should be adding, and I just check it. I use varying salinities on my mix anyway - depending on how much I've been topping off, etc.