As I've been tagged, my two cents...
As TMN has noted, more often than not I ride long distance stuff with a Deuter Bike 1 rucksack (20L, straps to compress the size down when you don't need it), for which I have a 3l Deuter bladder. Both excellent bits of kit. I sweat anyway, a bag on my bag doesn't make much difference, and unlike some I prefer the weight on my back rather than my bike. I don't have the balance to ride and drink from a bottle so a bladder is much better for me in that respect. Also a very handy convenient way to carry a lot of fluid- lighter than four 750ml bottles, and you don't need to keep shopping or topping up en route. I do have a Carradice SQR bag, but with anything in it the bike starts feeling more like a truck than a sports car. The bladder and tubing is well-coated, and does not taint (unlike most of my bottles).