Yes, Sam, it has appeared here previously, and I have fitted some for friends an tried them myself. The design is an old one going back to the late 1970s/early 1980s here, and they used to be popular in bright colors when fitted to BMX bikes.
Sadly, I found them lacking in some ways. While they do help prevent oily chain "leg tattoos", they have some other characteristics that make them less than stellar. For a full recap of the discussion, start here:
http://www.thorncycles.co.uk/forums/index.php?topic=4154.msg18928#msg18928For what it is worth, I have found a bash guard does a really nice job preventing all the usual chain -> leg oily transfers. If it happens on-tour, the most effective means for removal is the interior of a down sleeping bag, the more expensive the better!
Don't ask how I know this.
All the best,
Dan.