Did my first ride yesterday with my new (it's also my first) HydraPak Delta that carries two litres and has little storage capacity. Since I have never done a serious ride with something on my back before, I almost anticipated some pain. However, despite the distance (160k) I didn't feel any different during or after the ride compared to a Pak-less ride. It was a pretty cold day though, 5-10C, so I had a couple of layers between the straps and my shoulders so the final verdict will have to wait a month or two. It was easy to drink from and the water tasted fresh even after seven hours.
One reason I got it is because I do rides of that distance every now and then, and together with two large bottles I had enough water for the entire ride.
/Csson