i have just used the new camelbak racebak in the 20mile Duddon fell race in the Lakes and it was excellent. after the first couple of miles it was a bit unusual but soon got used to it but the main advantage is that as you use the water it gets lighter and then you do not notice it is there. once the water is used you are not stuck with a bottle or a pack on your back
didn't get too many strange looks but i did feel like quasimodo at first but i have to say it performed better than i thought.
just a tip though. make sure you get all the air out first otherwise the sloshing will drive you mad