Despite Amazon's stock picture of a random third-party charger this is in fact an original, official UK Nintendo DSi Power Supply/Charger (See my customer image) which is suitable for the DSi and DSi XL. However it is NOT suitable for the original DS or DS Lite, although if you are looking for an official DS Lite charger it is also available on Amazon here:-
Nintendo DS Lite Power Supply (Nintendo DS)(Although note that it also uses an incorrect Amazon stock picture!)
You may be able to find DSi chargers cheaper on ebay, but many are clone counterfeit chargers which look just like the originals but have not been tested to European safety standards and could be a potential fire risk. Personally for a few pounds more I would much rather buy an official Nintendo charger which is exactly the same as the one that comes in the box with the DSi/DSi XL.
Overall I give this item five stars, because it does exactly what you expect it to - charge your DSi/DSi XL. Amazon also has the cheapest price I have seen for an official DSi charger (I dont know why Amazon require a 'fun' rating for a charger, but I have again given it five stars because if your DSi has a dead battery you can't really have a lot of 'fun'!)
One final note, if you want to order the official Nintendo charger make sure to buy it directly from Amazon.co.uk (or Amazon's Preferred Merchant 'Indigostarfish') As Amazon marketplace sellers might be selling third-party chargers due to the incorrect stock picture.
<EDIT> As of 2012 Amazon no longer appear to stock the official DSi charger (Or the DS Lite charger) If you buy a charger from an amazon marketplace seller its a complete gamble as to what product you will actually recieve - most likely it will be a third party/generic charger.