They include these with loads of papers and magazines nowadays, I was surprised you hadnt seen one before lol.
They are a scam through and through, although not the biggest of scams, in that if you do phone up, you get a prize, but its always something lame, nobody ever wins one of the big prizes.
They have been on Watchdog a few times, and by law now, companies running the scheme, must print details on the back of how you can claim by post thus avoiding the expensive phone method, but there's nothing to say that it wont "get lost" in the post, or be returned undelivered.
I did used to every now and then send one of these off to the postal claim address and got loads of (mostly) cr@p back. The only good thing was a £10 M&S voucher - loads of free holidays (if you book 4 other people) and other nonsense!
we "won" a holday a few weeks back now after a free prize draw we entered... sounded pretty genuine till i rang the free fone number an was told i had to book on other ppl, and then attened a seminar in warrington... may of been genuine lol but lost site of that lol