This is turning into a meeting of Water Bottle Hoarders Anonymous! "Hi, my name is Richard and (telling pause...) I collect water bottles."
I reckon that they are the unassuming but absolutely essential bits of kit. They often reflect the fads and activities of a given time but rarely get 'moved on' because of their usage. Having started with my Sigg, I did try the collapsible/soft bottles and bladders because they were 'on trend'. I didn't like the taste the bladder I had produced in the water in it and found the floppy ones leaked, so I returned to hard bottles. My two Finnish 1 litre ones are excellent - currently - but who knows how I'll be tempted in the future?