I was just having a conversation with a coworker about her Kleen Kanteen the other day. She used to fill up her Nalgene from the chilled water cooler in the morning and sip from that at her desk throughout the day. She says that after she switched to the Kleen Kanteen, she thought that the water stayed cooler longer.

However, being uninsulated, I doubt that this would be the case in other situations, like strapped to the outside of your pack with the hot sun beating down on it.