It's amazing this thing, they've managed to find a way to make products that are switched off actually draw power
Nah, I think it's a good thing. A lot of stuff won't switch all the way off unless you unplug it or use the switch at the wall. Most people are too lazy to do that so this, really, is the next best thing.
You'll notice the biggest savings on things like plasma televisions which can draw a ridiculous amount on standby. My friend measured his and it drew a wallet slaughtering 400W on standby
So yes, if you've got something like that it's definitely worth installing if you don't want to crawl around under the TV to unplug it.
Other things like printers, digibox, monitors, computer, video, DVD player etc are actually not too bad these days. Many of mine register just 0.5W on standby, so the manufacturers have been pretty quick to fix the problem products sapping power on standby.
Go by the '1W for a whole year costs about £1' rule if you're wondering if it's going to save you any money.