Absolutely 1000% rodi units are worth it. If you factor in the time and money you spend on not only the water, but gas to get it, and your time to go get it, an rodi unit pays for itself relatively quickly. The best part about making your own water is you know exactly what you're getting. Water from your local fish store may not be as pure as you want as they can forget to change filters. So to answer your question, yes. An RO DI unit, in my opinion, is the one piece of equipment I believe every reefer should own. As far as what kind, that's going to depend on your city's source water, and how much water you're going to be making. A 75 gallon per day 5 stage is the most common