looking at the efficiency calculations above scared me!! mines a modern gas combi boiler, with piped gas, last time the boiler efficiency was measured during a service it came out at 85%, not the 50-60% quoted above. I get instant hot water whenever I want, and the shower is nice and powerful. don't know how it compares financially to other types but I love it!! had oil heating in my parents house and hated that we always ran out of water, or couldn't have a shower without having to wait an hour for the heating.
have absolutely no facts and figures to back it up, but I always thought that an electric shower was fairly inefficient due to using an electric element?