Minimal, light, great.
I bought this after wandering the Outdoor Retailer trade show in Salt Lake City. I went by the booths of every hydration vendor, and looked at what they had in the way of minimal packs. I was looking for hydration packs for marathons and ultrarunning events. The CamelBak Classic was by far my favorite.
There are two key advantages to this pack. Firstly, the bladder is super easy to refill without removing from the pack. The mouth is just tucked under a flap. This makes it ideal for race aid stations: you can easily unscrew lid as you approach, and quickly refill from jug etc. Secondly, there's really nothing to the pack that you don't want. There's a small zippered pocket on the outside that is big enough to hold a few gels. And a minimal bungee that can be used to secure a rain shell or a long-sleeve. Larger items can also be slipped into the bladder pocket. All the straps are mesh and highly breathable.
I liked the design so much I bought two, so that I can leave one at bag-drops/checkpoints and just swoop in and grab a full pack.