Please read his reviews on Calpis Flavors or Bottled Hello Kitty Calpico Water and his most recent excellent video review on Calpis Concentrate. There is Wikipedia entry on the drink here.
My take on the can version of the drink is this, if you take water and you add milk to it, it doesn't matter how much sugar you add it's a gross concept... However, it's a seriously popular drink and I don't mind downing the odd can when I'm having Ramen or Katsu-don at my favourite Japanese Noodle Bar, Ichi-ban Boshi (That's where the can pictured is from). It has a very sweet taste, and although I'm sure it has some kind of milk in it, I wouldn't describe the flavour as milky at all.
The drink is very refreshing and as asianbev has demonstrated, comes in a variety of flavors, including; strawberry, grape, guava, mango, pineapple, orange, melon, peach and others (sited from Wikipedia). The official Calpis website has a US English version that contains info on their various products.
The flavour is kinda tangy and probably created by the inclusion of lactobacilli which is some kind of yoghurt culture/bacteria I believe, although I wouldn't necessarily have noticed the yoghurty taste. It's interesting enough to make me want to drink it again, and unique enough to ensure that I won't be buying other brands.