I use my Camelback while road riding quite a lot and find it very useful. During this recent super-hot week here in SoCal I've been packing in as much ice as I can, then filling the spaces between the cubes with cold water.
So, the iced-Camelback plus a frozen Polar bottle, plus a frozen normal bottle keeps me on the road for four or five hours without having to stop. I also use the Camelback on CA Double Centuries, where checkpoints can be 35 - 40 miles apart (about the same distance as controls on brevets), and even two large bottles aren't quite enough on a really hot day.
Of course (as noted above), I always use it when I ride my mountain bike!
Rick / OCRR