My GF prefers a Camelback for convenience and ability to carry any extra gear plus she's never gotten real comfortable taking her hands off the bar to drink with a bottle. I've never liked have any added weight on my back since I've had lower back issues since i was young.

FWIW Bobby Julich used a Camelback fillied with ice water for TT including some great performances at the Tour. Some of his team mates may have followed suit. Claimed it saved time by not moving your hands out of an aero position and the ice water helped keep the core temp down on hot days.