I tried this on my previously uncleaned oven and it's bloody fabulous. I read about it on here and my racks look new and the bottom of the oven is clean enough to eat from. I need some more though as I want to unscrew the glass from oven door and do that.
For those of you who haven't tried it, it's an oven cleaning system with a great big plastic bag you put your oven shelves in. I paid about £3 from wilkinsons.