Article in the Sunday Times couple of weeks ago talking about how some of these things are a waste of money. Didn't read it myself but might be worth looking up.
I think a lot depends on the "variability" of movements to and from home. The more 'in and out' you are the more benefit you can get
It's great.
I got it free cos I work for BG and went on an installation course for HIVE.
I think it's free if you get your gas and electric from them, might be wrong though