I use one on every tank that I have. The only con is the initial cost, IMO. The pros are balanced calcium and alk, other trace minerals that come from dissolved coral skeletons (strontium, magnesium, etc. that people don't dose in 2 part). Reliable and easy - I just give mine a once-over every few weeks or a month to check for bubbles and if I still have enough media. I would never run a tank without one unless it was REALLY small and I didn't have any true coral.