I've got a couple of the Explorer Canteen 1.0 litres (
http://www.drinksafe-systems.co.uk/products.php). I've used them for streams in this country and whilst travelling and trekking through India and the Himalayas (great for drinking the tap water over there!).
They're great. Dead simple to use. The water can be drunk straight away and they only get safer until they wear out (at which point they stop filtering water altogether).
Only thing to bear in mind is the water draws fairly slowly through the filter, so fine to drink from but takes a bit of squeezing to transfer the full litre into a separate vessel (for example when using it to cook or boil).
Amazing for the price really, a lot less hassle than a millbank bank or one of those suction thingamibobs that cost a fortune.