don't really notice the sloshing, but then again I've headphones on most of the time
the little pack really holds a lot more than would meet the eye, I use the two upper front pockets for a camera and Clif blocks, the lower front pockets for electrolytes and Clif blocks
the small rear pockets I keep my phone and keys in one, the other has a small fire kit, whistle, small knife and headlamp
the main pocket goes a 2 person heatsheet, Buff, beanie, gloves, windshirt, extra food, TP, small fak- all fit w/o any problem- there was still quite a bit of room and that's not even using the outer bungee setup (where it would be easy to stow a rain jacket)- I was wearing my windshirt the other day and easily fit in my R1 vest rolled up- I know my light down jacket would fit as well
I ordered a 1 liter Platypus Hoser (it also accommodates a bladder) for long runs w/o water sources (contemplating running across Joshua Tree NP in March)- it would steal some away some volume though
typical run loadout