I never drink from the fountains and I also drink bottled water at home. I only drink water (no soda, soft drinks etc) so I am pretty picky about it. When we lived in houston, the water was SO awful that we had a very expensive whole house water filter with an additional reverse osmosis unit under the sink for drinking water and THEN I drank tap. When I lived in NY I also drank tap until the one year when the reservoirs were low and they started supplimenting with Hudson water, blech. I could taste a noticible difference so we switched to bottled at that point. We live in an apartment now so bottled water for us again.
I'm actually glad that WDW switched to desani because as far as bottled waters go, zephyrhills was some of the WORST bottled water I ever tasted. Dasani basically purifies the water and then adds back a "flavor pack" specially formulated for various regions so that no matter where you buy dasani (they basically use local water to make it) it will taste the same. The flavor pack is basically minerals, nothing scary.
In case you can't tell, I am VERY picky about my water... but I don't spend tons of $$ on sodas and starbucks and all of the stuff that most people drink so I am willing to spend it on good tasting water.