I've had both success and failure with sling style packs. A belt is nice to keep things where they should be but some styles will always end up sliding around under your arm or in front. I'm not sure if it's the cut/design that contributes to this or the materials. I suspect that a strap which is padded and some what grippy won't have as much of a problem with this.

The other thing is that they're small and easy to over pack. They won't hold much and don't like to be heavy (it makes them want to slide around and under even more). I have one but never use it for more than a small water bladder and a bag lunch...anything more and it loses all practicality.