I just don't like the flavour of my tap water. Maybe it's just my tap water, maybe it's just me. Usually, I only care about flavour if i'm drinking it as refreshment. If i'm using it to keep me hydrated, I don't really care where it comes from.
As for taste tests, I believe ebaumsworld has a video of people drinking hose water thinking that it was $5 or so bottles of water. They all said it tasted great
(or at least those they showed anyway...i'm sure there are people who can taste the difference)