http://www.rei.com/p...i-flash-18-pack
got a new pack last saturday and used it hiking on sunday 
Loved it... it has a big pocket at the back (inside) for holding your hydration bladder (ordered one from goruck I will let you know how I liked it) I used it for holding two 1.5 L bottles... the back is lightly padded and once I buckled the two straps across the chest and belly it didn't bump at all... it was a very low profile bag... we hiked old rag (very strenuous hike with some small spots) and there was only like two spots where I had to take it off to get through (and those were spots where I scrapped myself going through so ANY bag would have been too big!)
I usually carry a string bag because I don't like weight on my back and I don't carry much... this bag had 3L of water and snacks and rope for climbing... and once it was on and sinched I could hardly tell I had anything on... the straps have a good padding but they are like waffle weaved or something so they are very light and breathable (of course it was COLD the day we went (tMAX was probably 40F tMIN was in the teens) so I don't know how the straps will feel in the heat)
used it over different layers of clothing (from just my tee shirts to all that and a hoodie) depending on the temp... and it was easy to adjust to the different thicknesses of clothes...
it has loops on the outside that I used a carabiner to attach my rope so it was easily accessible... worked well...
there is an opening in the top for the hydration bladder tube to go through... once mine comes I will tell you how i like it
