I don't have one on my bike because I don't like the way they look. It's no real hassle for me to go into my backpack for water at a stoplight, so that's where I keep my water bottle.
That's usually good for non-hot days. One thing I hate more than anything is a backpack on a hot day. Turns into the thing from the black lagoon down there.
That Minoura cage adapter is the one I was referring to. It's a metal on metal attachment.