Google "National Home Services scam." Then read this article: http://www.cbc.ca/news/canada/ottawa/story/2010/08/10/ot-water-heaters.html
This is an awful, awful business. They harass people into getting locked into terrible water tank "deals." ( The review from Jason S. already explains the math.)
In the past 3 months, I've been visited over 6 times by pushy and annoying salespeople. They keep coming back after being told time and time again I'm not interested.