I'm never without water; I don't understand how people go without it for so long and can only conclude that they must always have a dehydration headache! It helps living in the North though, where water is genuinely tasty; when I'm visiting London in particular my consumption goes down.
The worst I recall having was the tap water in Sicily, where it tasted milky (I guess a high calcium content or something). I like milk, but I don't find it particularly refreshing and can't really drink it on a full stomach even if thirsty. I found it really hard to stay hydrated in the heat there unless I bought mineral water. While I'm no great fan of the swimming pool water you get in some parts of the UK it is at least vaguely drinkable.
In usual circumstances, I have a bottle that I refill and probably require a fresh bottle every 3-4 months, but train travel is probably the main time I end up buying an extra bottle, so water fountains at stations would be a great thing.
I do recommend the "Klean Kanteen" stainless steel bottles. They retain a clean taste, don't ever leach anything into the water, and if they start getting a bit "musty" in taste a quick sterilisation with a kettle of boiling water (could probably use one as an iron when doing that
) brings it back to new again. They aren't cheap, but often it's a case of "buy cheap, buy twice", and the finest "eau du robinet de Bletchley" tastes nicer from one than anything in a plastic bottle.
You can also use them as hot water bottles when camping - I have done this!