i've heard of these jeans before (although i didn't know m&s were doing them). i've only seen them on tv mind, but they did look good when they were on. try them round the house for a couple of hours, see how you get on, just keep the receipt;)
I have never seen these jeans - but I fear that my figure is past the stage at which a pair of jeans will help.
I always wondered about the slimming underwear. You are basically covered from shoulder to knee in tight fabric that hold you in. What happens if you are out on a date and get 'lucky'? It would take you most of the night to get your underwear off!