On Demand Hot Water Heater Owners: Do you prefer your On Demand heater, or miss your old tank?

I want to hear from current users of On Demand hot water heaters. I have to replace my tank, and would like to make an environmentally friendly choice. But, I want to be sure that I will be saving energy as told by actual users of the product, and I want to know just how much convenience I will be giving up, if any. Thanks for your input.