After trying one of these at the Hawksmoor (seven dials) last year, getting the Hawksmoor book (which is where i got the recipe from) for Christmas, and a bunch of ingredients as a Valentine present from my wife, I've become slightly obsessed with this drink.
If you like ginger, then this is highly recommended. (Making up the syrup is a bit of a hassle, but once you've made up a batch, you can keep the rest in the fridge for next time.)
And a bonus tip - when you've made the ginger syrup, keep the ginger that you've used, leave it out to dry and sprinkle it with sugar, and you'll have a load of candied ginger, which is really nice. (If you like ginger, of course… but if you've come this far, then that is probably a given.)
[Update - the original link I posted to Channel4.com has stopped working - here is an alternative recipe that looks about the same. (I would put more than two ice cubes in though.) I would recommend buying the Hawksmoor book though - apart from all the useful stuff in there, its for a good cause too.]