Bought this to replace my regular sized backpack and a water bottle while mountain biking. The bag is small enough to not really notice you're wearing it, but big enough to carry some of the essentials to keep you going.
The bladder for the water is in a separate rear compartment and comes right out to easily fill, clean and dry out. The size is enough to get me through a good 10km ride (at least). The valve has a quick connect for cleaning / filling and works great.
There are 3 main compartments, and a padded area at the top to put your mp3 player in. I like this feature since it keeps the cord out of the way and out of my pocket.
There are a few compartments in each pocket, some with mesh zippers to keep things contained including my first aid; spare tubes, mini pump and multi-tool.
The chest strap has a whistle built into the clasp in case something ever goes wrong or you run into some wildlife.
Overall a good buy, I'm very happy with it and plan to use it for snowboarding this winter without the water bladder.