The Osprey Kestrel 38 is a really well designed rucksack. It comes with all the features I found wanting in some other models - the Kestrel does not compromise.
For example, the compression system does not come down on top of the side pockets, which could limit their capacity. The stretchy mesh pockets themselves great for water bottles - I am able to whip my bottle out and replace it without having to stop and remove the bag.
The hip-belt is the most comfortable I have ever worn - it easily trumps that on my 65 litre bag, which bruises my hips. It also caters for the fact that I have high hips, despite being 6 foot tall. One slight niggle is that being very thin I have to have the belt as tight as it goes when I am wearing only a T-shirt, so for younger wearers it may not be suitable.
The hip pockets are roomy and easy to open/close. I have my waterproof trousers in one and camera in the other.
As for the attachment straps at the bottom, they comfortably hold my inflatable mat, at near full extension. This can be a great place for the mat to free up space inside the bag for your tent, if you don't want that under the hood.
I would strongly recommend this bag for lightweight backpacking and day expeditions. My other use for it has been climbing day trips. For this I can just chuck shoes, harness, waterproof, etc.., in and not need to worry about packing it tightly.
All in all, I have been hugely impressed with this bag.