I am aware I probably sound like a right ponce but I can tell the difference between some brands. Personally I don't like the taste of Volvic. I quite like San Pellegrino but that's probably more to do with the fact that it's only slightly fizzy rather than the normal high level of fizz so it's nicer with a meal. Generally speaking though I drink either nasty fizzy pop (i.e. something with a flavour already in it) or squash / cordial with tap water or fizzy water from whichever supermarket I've been too recently depending on mood and how bad the tap water tastes / whether or not I can taste the tap water through the squash - as the tap water here also goes through phases of tasting like it's got far too many chemicals in it (i.e. it can taste like bleach / detergent, not that I've drunk either of those!)
Edit - interesting we've had a few mentions of a north south divide in tap water taste. I never had a problem with tap water when I lived in Nottm, Stoke or York but around here (Wokingham) it definitely has some pretty minging spells though it's been OK for months now. That said my level of water consumption has probably increased (in line with a decrease in beer consumption) as I've gotten older and (slightly) more sensible so maybe I'm just noticing it more?