I can't say I use mine strictly as a shoulder bag. In fact, I often wear it on the crook of my arm, but when my little one is with me- it is all about the shoulder strap! That is why I love it so much- it is light and so so versatile!
I use handles on my arm and the strap for my shoulder. And that's for all my bags, not just the city. The more stuff I have in it, the more likely I'll use it on the shoulder.
I love wearing my City with the shoulder strap. I love the way it slouches and also the ease of carrying bags on the shoulder. Also it limits me touching the handles and darkening them. Occasionally I'll wear the handles on my shoulder but I find it isn't as comfortable.
Thanks lordguinny - good point about protecting the handles from discoloration. I'm getting a light colored one, so that's another reason for me to use it mostly as a shoulder bag.
I use my city as a shoulder bag about 90% of the time, except I put the handles on my shoulder, not the strap. I don't carry a lot of stuff in my bag and I have found that it hangs funny when I try to use the strap, probably because I don't have enough to really fill up the bag.
I use both, too. Lots of times I don't even realize which one I'm using until I look down and see that I've thrown the strap over my shoulder, or that I'm wearing it in the crook of my arm I love the versatility of that!!!