UPDATE (January 2013)
I wrote this review well over a year ago, and in reference to the then current price for this (less than nine pounds!). At the time I wasn't thrilled that Amazon had kept the price point the same for a basic mains plug (not supplied with 'baby/All-New' Kindle... or later Kindle Touch/Paperwhite) as it had been for a plug + USB lead replacement charger bundle, but was not totally put off by the price point.
Having just gone to buy a replacement plug for my mum's Kindle and spotted the change in price, I have to agree with more recent reviews: close to twenty pounds is far too much to pay for a simple USB to mains plug! Particularly when there are lots of well-reviewed products that do the same job for far less. Albeit typically in black!
Rating dropped from 4 stars to 1
Amazon have changed their power adapter. You now receive a plug ONLY rather than a plug and lead. For the same price.
Having just the plug makes this adapter ideal for use with the new Kindle or refurbished versions of the Kindle Keyboard wi-fi/3G (all of which ship with a USB lead only), as the end of the USB lead you already have slots into the plug, converting the lead into a mains charger. But it's not so great if you have a damaged lead/plug and aren't sure which bit needs replacing or have mislaid your power supply - as you will need to buy both this plug and a USB lead. Even at the current reduced price for the plug, buying the separate plug and lead is more expensive than the two combined used to be.
I find charging my Kindle via the mains is significantly faster than drawing upon a USB port for power, and that it's easier to keep reading with the Kindle plugged in this way than with it attached to my laptop/desktop, so in this respect I think an adapter is worth having. However, the pricing for the own-brand plug is a bit steep, especially when compared to the cost of the low end Kindle which went on UK release earlier this month. Even at the reduced rate, the plug is more than 10% of the price of the Kindle!
As the new Kindle has the same size USB plug as the Kindle Keyboard models (and many mobile phones) there are plenty of third party solutions, including a range of leather cases which come bundled with chargers. If you're lucky, your existing mobile phone charger may do the trick.