So far my impressions are pretty good - my typing speed and accuracy seems no worse than usual, e.g. still crap, it doesn't seem to make pressing on the keys any harder, and it's staying in place without any adhesive etc. Considering it only cost a couple of quid including postage I'm very pleased. No idea how well it will last, of course.
if anyone else in the UK wants to try one, the eBay listing is here:
While I bought the clear type, you can get them in various colours with the key labels on the plastic. This might be very useful for someone who often needs another language or keyboard layout - I just checked and you can easily get e.g. Russian, Hebrew, Japanese, though some of those are more expensive since they come from other countries. Lighter if not quite as convenient as a plug-in keyboard.