I bought a Lowe Alpine daypack online and unseen, it was nicely made and good to wear, but what the ads at the time didn't reveal was that everything inside was a bright orange colour which I couldn't live with as I only like muted, earthy colours. (I think the ads were changed after I took it up with LA.)
I returned that and bought instead a Vaude one, again online. This is well made too but to be honest the shoulder straps could do with a bit of padding, while I've readjusted it umpteem times and it still doesn't feel quite right when I'm walking.
Therefore I second what the previous two posters have said about trying them on.