As soon as we have our own home, we'll be buying one pronto.
DH has done a lot of reading about them, and frankly, I can't think of any cons. There are several types depending on your usage and if you want to add solar to it, and whether you want an electric or gas pilot.
The large ones can run several showers or the shower and washing machine, etc. They heat the water on-demand (basically it goes through a very small pipe with a flame on the pipe so that as it passes, it heats up, rather than heating a huge pot full like a tank heater does.) It costs much less money than a regular heater and you never run out of hot water!
Early intervention specialist and parent consultant since 2002.