All the compartments are great, including the hip belt pockets. I recently went on a 5 day hike and had no problem fitting and organizing all my gear. I love the top compartment and side ones for their capacity and easy access - a must when you need to have certain gear and snacks easily accessible.
This is the most customizable pack for fit that I have ever seen, you can really dial it in for a perfect fit. I should mention, at first I had an issue with the hip belt slipping. The hip belt has a pad that extends and is attached via velcro pad inside the strap. The actual hip belt strap attaches to the extendable pad, so all the pulling force is actually applied to the velcro. One side kept slipping on me (which was very frustrating). I finally realized there was a piece of extra fabric covering most of the velcro pad, so it didn't have the needed holding power. After removing the extra fabric off the velcro I have had zero issues with it slipping.
People have mentioned they want a hip belt strap to convert the brain pack into a day pack... I have used the full pack as a day/summit pack and greatly prefer it over a small uncomfortable lumbar pack. The compression straps suck everything down nicely and the lighter the load, the closer it feels to real "Anti-Gravity". Plus I like keeping my hydration reservoir with me at all times, cant do that with a lumbar pack!
Overall, the pack is super light, comfortable, functional, and versatile. I highly recommend it.