After a few trips through Arizona and New Mexico it is a must have item. I elected to purchase the Mark8000 and it general lasts 2-4 weeks between salt treatments. If you buy one make sure that you get the test strips available on Amazon. That way you will know when to refresh the unit.
A Mark 10,000 obviously provides more capacity but it was heavier and a bit more awkward to set up when I am on the road so I selected the next smaller size.
Also if you are going to be long term at one location Culligan often rents the larger units by the month and provides you with a fresh unit each month if required.
Mark 8000
Lots of good threads on this subject. Just Google RV water softeners and you will find plenty of info on iRV2.