In reply to James Malloch:
I've got the 20 litre OMM one that I use for mountain marathons and running in and out of work, it is great but because of it's light weight nature you need to fill it for it to carry well; I often find muself putting an old towel in as well as my work stuff just to fill out the space. Also ensure that phones / wallets / keys etc are in a small bag inside it as the zips can come undone, (this has happened once to me,) so you don't want stuff spilling out. (I think I've countered this by pulling one zip tag up, over and down the pack rather than meeting both zips at the top, if this makes any sense!)