I can only look at the guys I work with and see that among the 20 or so full time messengers here, one uses a chrome backpack and the extra time he might need to get in and out of his bag is made up for the fact he is also a Cat 2 racer.

My boss also prefers a backpack over the sling as he says it is more comfortable and even more water resistant.

I think it all comes down to preference and what one has to carry on a daily basis... if I am travelling light I don't use my Chrome bag but prefer a small rock climbing bag.