They are quite common in the UK, and have been the default for a couple of decades now. They are definitely more efficient, compared to a tank, as well as taking up far less space. You just need to make sure the model you choose can keep up with the demand. Nothing worse than a tepid shower.

The only situation I can see them being worse is with solar. If you have solar panels then dumping excess power into the water heater is a great way to not waste it. With a combi boiler, that is no longer an option.