I was surprised how stable this is on me. I'm 6'2" and 70kg and expected this to move but once you have t straps right it's very good - even on a quick run like a 45min 10k.
You can tighten straps around the bladder while you run to stop the slosh - but as long as you get all the air out before the run you wont notice the water moving.
Once down to about 1l of water it's almost as though it isn't there.
Great running pack. Bought for 2hr+ runs - comfortable, cool, extremely light and practical. Lots of accessible pockets for gels, keys etc and very secure bladder, with well thought out compression straps for stability.
Merlin delivered very quickly to Australia.