I have had the Camelbak "sipper" bottles leak a little when put on their side or shaken vigorously. When I'm carrying the weight on my back I don't like the weight of the bottle or the sipper lid as compared to a simple widemouth 1L AquaFina bottle.

I find the Camelbak sipper to be excellent, however, for use in the office & classroom. When delivering a lecture or working at my desk it is nice to be able to take a sip without opening & closing a screwcap, but still have some spill protection.